Vauxhall zafier 56 plate ?

Hi I ave a Vauxhall zafier 56 plate b. My fan works fine but not much air through the vents. No air throug windscreen vents I've taken all the panels off on the drivers foot well to locate the white wheel which moves the arms for the vents. I've followed the arms right up in to the dash. I've noticed that the arm has came of the pin that moves around the windscreen vents.. I'm not sure how to get it back on has having now room. Can you please help