Once on the parapet there are a few different ways to grab on to the lamp post:
Stand facing the door on either the left or right side of the light post. Free run up the wall to the left or right of the lamp post and complete an extended jump by pressing (A - 360) / (X - PS3) and aiming in the direction of the light post at the top of the run. You should then grab on to the lamp post and be able to continue climbing.
Stand on the little wall directly underneath the lamppost. Hold RT (360) / R1 (PS3) and jump by pressing (A - 360) / (X - PS3). This will allow Altair to grab onto the post and continue climbing.