ForeFlight supports the Merlin's ADS-B (TISB/FISB) and GPS features right out of the box without modification. Unfortunately, FF uses a proprietary communication for AHRS and it only works with the Stratus ($900).
Stratux's (Merlin) developer has created a special version which adds support for FF but its experimental. Once applied the AHRS will only work with FF. GPS/ADBS will continue to work with supported apps.
Once downloaded to your PC or Mac, change your PC/MAC WiFi connect to the Merlin/Stratux WiFi. Using your internet browser point it. This is the built-in Merlin/Stratux status web page. From here you can upload the file.