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/