From http://www-bsac.eecs.berkeley.edu/~cadence/tools/import.html

Importing files

Step 1: Destination Library and Technology File.
Cadence requires that you have a both 1) a design library that will store the imported file and 2) a technology file/library, which defines the layers. The design library and the technology library can be the same library, or they can be different libraries.
WARNING! If you import a file into an existing library, you might overwrite existing designs. If any cell in the GDS file has the same name as a cell in the destination library, Cadence will overwrite the existing cell with the imported cell. Cadence will not warn you. I fouled up a circuit layout once because of this. I strongly encourage you to always create a new library, in one of the following ways, before you import a file. This is your only warning.

1. Start Cadence
2. Two windows will appear. CIW (Command Interpreter Window) and Library Manager.
3. Technology file

• If you don't have a technology file or library, you need to create a new one:

          CIW -> Technology File -> New..
          Technology Library Name: your_library_name
          Load Existing Technology File: cdsDefTechLib

(You can choose a different existing technology file if you want, but cdsDefTechLib is recommended if you are going to create a completely new technology.)
At this point, I believe that you have to quit and restart Cadence in order to be able to use your new technology library.

• If you already have a technology file, but you want to create a new destination library:

      Library Manager -> File -> New
      Library Name: your_library_name
      Attach to an Existing techfile
      Attach to Technology Library: your_existing_library_name  

Step 2: Import File

Importing a GDS file

  CIW -> File -> Import -> stream
 

In the “Stream In” or “CIF In” window

  Input File: your_input_file_name.gds 
  Library name: your_library_name
   

If you are using an existing technology file for which you do not have write permission:

• Click “Options”
• Check “Skip Undefined Layer Purpose Pair”

If you have a layer map file:

• Click “User-Defined Data”
• Layer Map Table: your_layer_map_file

Once PIPO STRMIN completes successfully, you need to refresh the library manager

  Library Manager -> View -> Refresh 

If your imported file has layers that were not previously included in your technology file, or if you created a new technology file, you might want to edit the color and stipple pattern of the layers. There are two approaches:

Step 4: Edit Layer Appearance

• Edit your display resources file (DRF)

Advantage: This might be the easier of the two methods.
Disadvantage: Since your DRF is unique, other users won't see the same layer colors unless they copy your DRF. If you are sharing layouts, this can be a problem.

  CIW -> Tools -> Display Resources -> Editor
  Application: Virtuoso
  Tech Lib Name: your_library_name
  Layers: LSW

Select a layer and edit the color and pattern as you desire.

  File -> Save

If you call the file display.drf and save it in your Cadence start directory (e.g. ~/cadence/display.drf), it will automatically load whenever you start Cadence. Otherwise, you will have to load it yourself (File → Load) later on.

   
 * Edit your technology file

Advantage: You don't have to keep track of which display.drf file. you are using
Disadvantage: It is more tedious to change the layer appearance.

  CIW -> Technology File -> Edit Layers
  Technology Library: your_library_name

Select a layer and click “Edit”
To change the color and stipple pattern, choose a different display packet under the Display Resources heading (e.g. METAL, goldX, …)
To edit another layer, unselect the current layer in the “Layer Purpose Pair Editor” window, and select the next layer. A selected layer is surrounded by a thin box. It can be quite difficult to see if a layer is selected.

A layer map file tells Cadence how to convert between layers in a Cadence layout and layers in a CIF or GDS file. More detailed help can be found in the Cadence help on the “Translator” product in the “Design Data Translator's Reference.”
Note: You do not always need a map file.

If you do not provide mapping information in a layer map file, the translator creates layers with names based on the original layer number. For example if the input layer number is 20, the Design Framework II layer name is L20. The default layer purpose is drawing.
GDS Layer Map - Text File Format

      #Cadence layer    Cadence layer    Stream layer    Stream data
      #name             purpose          number          type
      #
      metal             drawing          13,10,18        0-63
      poly              drawing          40              0
       

Notes

• Cadence layer purposes are listed in the LSW (layer window), example:

          dg = drawing
          d1 = drawing1

• Generally, I use stream data type 0.
  
• If you specify a layer map table in the “Stream In” form, Import - Stream translates only the layers that are listed; it does not translate layers that are not listed in the mapping table.
  
• Stream layer numbers must be integers between 0 and 255. Numbers 1 through 127 are user defined, and 128 to 255 are reserved for system definition.
  
• PIPO can translate any of the 255 mapped Stream layers, but due to GDSII format constraints, PIPO can translate only 64 different kinds of layers in any one translation.