I need help in ins creed 2 for the xbox ?

I get to the part were he woke up for the first time from the animus and he goes And helps the girl with something. He starts to hallucinate then he pes out and thinks he's dreaming . He is back to the town but its gray and he people Are like glowing white. Then he says how he has to fallow this person. So I chase them to this tower And the door closes and I need climb the tower.... ( here's were I get stuck) their is a platform with a door way then a wooden rood with a lamb on he end of it . And for my life I just can not jump on top of the wooden thing . And every time I try I end up falling of off the tower and dieing . I really help