There is a building with sandbags on top of it, climb up the ladders.

Then climb on the sand bag nearest to the ladders and sprint jump on to the lamp-post.

Then if you look left you should see a blue air vent box, go as far back on the lamp-post as you can.

Sprint jump on to the air vent box and there you go.

If you want me to show you this cheat contact me on my Gamertag.