1. Ensure the Grant record is linked to a transaction code

Then, if the Budget tab is still grayed out:
  1. In Configuration, select Business Rules
  2. Select Grant and select the appropriate Transaction code from the Use Values from, drop-down
  3. In Records, Grants, open the Grant record
  4. Select the Projects tab
  5. Select a Transaction code table entry from the drop-down menu at the bottom.
  6. Click the save icon in the Grant record.  The Budget tab will now be available to select.