It has a park and drive position, the photo is the drive position, I'm not convinced the belt is in bad enough condition to be the problem.
Very frustrating because it is just so simple.
Engage drive
Press handle grip to activate cable clutch.
Pulley moves outwards and engages the outer clutch plate (the one with the 2 holes)
That then drives the pinion gear which is engaged with the ring gear.
Rear roller should turn.

[Linked Image from i52.tinypic.com]