I found when I did this mission that the tracker refused to point me at the right place, instead telling me I needed to go back down and exit the Silo. I was able to finish the mission anyway by going to the front edge of the roof and watching for the bars to go up on the tracker, pressing G at the correct spot. So don't get confused if your tracker steers you wrong, you can still complete this mission if you find the right place on your own. It took me a while to realize that though, so hopefully this comment helps anyone else who ends up with the same confusion.

In order to access the top of the block, you have to activate the console (5845,-11990). Then, some "green platforms" appears on the left of the console : - Basic stairs + elevator for step 2 - Path with all the odd platform blinking + elevator for step 3 - Stairs with all the odd platform blinking. Finally go to the front edge of the roof and validate.