Here's a description of using the Futaba 4-channel in the sim (FAQ below). If you want to fly w/ an airplane that has retracts, then start/copy the FS One Tx for that airplane (e.g. the P-51) in the Tx group mentioned in this FAQ:

http://www.inertiasoft.com/fsonefaq/#Q1027-FS1

Take a look at the FS One Flight Manual for the airplane/Tx that you start with. The gear is usually tied to Key-4. Click on the "Flight Manual" button to see the assignments.

I hope that helps.

Michael