Process to Enable DFF for a Lookup Value set

Create a DFF

Login to System Administrator
Application -> Flexfield -> Descriptive -> Segments
Search for Application = Application Object Library
Title = Common Lookups

You can see referenced field as "LOOKUP_TYPE"

Now if you want to enable your lookup, you need to uncheck "Freeze Flexfield Definitions" checkbox on the top left the DFF Segments screen.
this enables you to enter new DFF entries.

Now, enter create a line under Context Field Values 

Under "Code" enter your lookup ex: XXSYK_DEDUCT_ELEMENTS

Click on Segments

Enter the number, name, prompt, column and value set(if you have one) 

save and compile the DFF.

Now, switch the responsibility to Application Developer to see the DFFs enabled in lookup

Open the common lookups screen and query for the lookup name you have given under "Code" of DFF setup.

and thus your DFF is enabled.