In FIMS versions 14.21 and lower, that field is unavailable for direct edit.  This makes it seemingly impossible to add or modify the value.  Fortunately there is an alternative way to access that field.
We anticipate a fix in a later version of FIMS, but until the change is made, do the following:
  1. Open the Grants Module
  2. Click on the Grantee supertab
  3. Find the Grantee record
  4. Click on the Relationship tab.
  5. Find the Contact Person you wish to modify.  Note that the relationship type should be GC ( or whatever your default grantee contact relationship value is). 
  6. Scroll to the far right.
  7. Modify or add a value in the Addr Type column and click Save.
  8. Check the Contact Persons tab for the changed value.