You have ordered your liftpass online and you have the withdrawal on site ?
Here is the list of withdrawal points in the resort.
Go to the nearest point (you don't need to select 1 precisely)
Chamonix Mont-Blanc
vallorcine
argentiere
Les Houches
Saint-Gervais
megeve
Courmayeur - Italy
Summer passes & E-tickets
MONT BLANC Rapidcard