Originally Posted by willwren
It'* just a non-grooved wheel that the BACK of the belt rides on, right above the water pump.
It'* a pulley on a spring-loaded mounting. Looks like the other pulleys, with a regular hex-head bolt sticking out the middle. Put the right metric socket on it, attach the longest handle you can find, and turn it to loosen the belt. You're fighting against the spring so you have to sort of hold the wrench handle with one hand while you lift the belt off the pulley with the other.
Note that it'* _not_ necessarily counterclockwise to loosen; in fact the alternator belt on my SSEi is loosened by turning the tensioner bolt _clockwise_.
You don't have a label on the engine anywhere that shows the belt routing? Mine is on a small cover near the belts...