It can be a bit tight! No real pearls of wisdom, but the pulleys can be jiggled slightly clockwise & anti-clockwise, even with the pins in place. There is also a little wriggle room with the crank pulley. You will probably find that once it's all alligned, it will slip on relatively easily.
Of course, you may have received a 'bad' new belt. Have you tried refitting the old belt? Also, have you overlayed the old belt loose over the new belt to compare sizes? Might lead somewhere?
Good luck!
