Go on top of the building with the boiler, once up there jump onto the ledge hanging from the sky.

After you are up there jump onto the crane. After that, when you are on the crane, you can jump onto the big sign or run right across it to the right and jump onto the window cleaning ledge.

Then there should be a small little ledge going in to the building.

Kneel down and there should be a little square - go inside it and that's you!