When you work you way through the apartments you will head onto the balconies and climb up the plant fixtures to the next level. The first beam you come across won't count for this achievement so continue on and you will climb another plant fixture to the next level, at that level you will see another beam straight in front of you, cross that to gain this achievement.
Be aware if you fall you will need to reload the save and if you choose to cross you won't be able to collect all wishes, so do this at the very end of your achievement hunting.
If you need more help you can find this on my video below, it will start at this achievement: