If your MCP Pro is functioning in other simulators but not FSX, try the following fixes:

Ensure you have both Service Packs installed (these can be found here: https://www.microsoft.com/Products/Games/FSInsider/downloads/Pages/default.aspx)

Depending on your permissions settings, FSX may need to be run in Administrator mode to detect your hardware. 

This can be done by right clicking on FSX, and selecting "Run As Administrator"

If these solutions do not work, please submit a ticket so we can assist you further.