Face the Marvel HQ Building and head to the left, go into the alleyway, the ramp starts on the OTHER building. You'll know it when you see it, it's RIGHT next to one of those SHIELD Hub controls things.
The ramps scale up on one building and then you jump onto the other one and take it up to the roof on more ramps.
http://tmproductions.proboards.com/index.cgi? GT: Doktor ALUCARD PSN: DoktorDare