ForeFlight assumes the arm value increases as you move aft in the aircraft. To get your aircraft W&B profile to work within ForeFlight, you will need to reverse the associated sign (+ or -) with any datum referenced arm value.
For example, if your cockpit seat currently has an arm value of -62 inches, that would change to +62 inches. If the empty moment for your aircraft is 13,751.3, that would have to change to -13751.3.
Doing this will cause the Loading Summary to appear incorrect since the + and - will be reversed, but will otherwise look as expected. The graph and summary section should appear normal.
The Dehavilland DHC-2 is an aircraft that faces this situation. Using the Dehavilland DHC-2 as an example, if it has a bench seat with an arm value of -62 inches, that would need to be changed to +62 inches. If the empty moment for the aircraft is 13,751.3, that would have to be changed to -13751.3.
Image 1: Aircraft POH correctly entered arm values that are decreasing in value in terms of aft movement.
Image 2: Adjusted Arm values in a typically decreasing arm value aircraft (changed + and -) values .