I have located the answer - I had been doing all my work in the logical model . Turns out by opening up the physical model, you can access the grant listings.
there is no support to import just grants in DDL file - objects and users/roles should be in the same file. As workaround - you can generate DDL for your physical model and add grants at the end. In import and merge part - deselect everything but users and then do the merge (only for users).