This can be done quite easily (I had this set up before I went wireless to my iPad mini).
There is a whole thread on this in the forums (search fsx).
Basically 2 programmes are required:
1. Fsx2NMEA (to get Fsx to output the data into a format that Skydemon can read)
2. FSUIPC - a fsx AddIn that configures the ports used to transmit the data used by Fsx2NMEA
I can't remember the settings (as I've changed them for my wirless iPad mini), but these are discussed in the fsx thread I mentioned earlier.