Look into your Forza settings and set your button layout to default. This, in my experience, made the clutch work via the pedal.
As for the shifter, did you calibrate? Do this by turning on the wheel only (no ps3/PC/xbox mode), have shifter in neutral, then press and hold the tuning and start button at the same time until a 6-- is on the display. Once this is done, move the shifter into Reverse, press start.....then proceed to do this in each gear (gear, press start), press start after 6th and the wheel should turn on to Xbox mode.
Of course if you have done this and still nothing, I can no longer assist as I do not know, good luck/
All you should have to do (how I did it in PS3 mode) is-Posted my experience in another thread but thought it might be usefuly here...when my problem is resolved I will notify yall
Contacted Fanatec via e-mail and they advised me that I cannot sync gearshifter in Xbox mode. It HAS to be in either PS3 mode or PC mode. So I drug out my cables for the PS3 and hooked it up and put the wheel in PS3 mode and then synced with no luck. Im still missing 5th gear. Dont know if I have to reset the whole wheel and try it over since Ive already synced it the first time in Xbox mode....I guees I would have been out of luck if I didnt own a computer or a PS3 if I wanted to use my gearshifter