If you are trying to convert an OAuth button, Formula Builder does not recognize it. The current recommendation is to convert to a non-OAuth button, create the formula with Formula Builder, then change the button back.
To Edit a Formula Field:
Click the Congatab.
In thebox, select the master object specified in your Conga Composer button or link.
The Master Object is the object where your Conga formula field is placed.
In the Formula Field area, in the Name box, select the formula field to update.
The converted Conga Composer button or link appears in the left pane while the formula field it was converted to is in the right pane.
Disable the Show Converted Formula checkbox to view the button in URL format.
The Conga Composer button opens in edit mode in a new browser tab.
Edit the button and click.
Return to the Formula Builder and in thedrop-down menu, select any other button, and then select the original button you edited above.
This is similar to a browser refresh (alerting Formula Builder that changes were made in the button) but allows the Formula Builder to remember your place in the update process.
Formula Builder notifies you that the button and formula field don’t currently match – click.
Clickon the dialog box confirming the formula field has been updated.
The button and the formula field now match exactly and the updates are complete.