I have same problem with a zanussi TD520 dryer. What is worse is that I replaced the belt a few years ago, I remember it was a pain to do and took me an age to work out how to do it - do you think I can remember how I did it? - must be old age.
I think you have to loosen the drum so that the belt slips on over the front (where it meets the door). Your model is probably similar. Good Luck
B Hunter
January 2006