Paul: If you have used the belt tensioner to mount the belt, disregard...
From the ISB Operation Manual:
"The belt tensioner adjusts with a 3/8th or 1/2 inch square drive and a long handle. It winds in the direction that the tang is bent over the tensioner body. To get the belt on- rotate the tensioner to wind the spring tighter.
Applying excessive force in the opposite direction of windup or after the tensioner has been wound to the positive stop can cause the tensioner arm to break."
With the correct belt, mine went on easily once I had a long enough handle to move the tensioner...My tensioner moved clockwise to get the belt on.
Hope this helps.