I have to take time to orient myself with the map; it's been a while.
You want to reach the south exit at round walkway building and the blue button panel.
I am not sure if you need to turn walkway at the control room to access the south exit of the round walkway building at this point in the game but since you're following the walkthrough - maybe you turned the walkways differently at one time.
Check Graham's map here.
The control building in this map is the square with 3 dots below the round building.
This is the tower where the control panel is located.
You want to go to cliffside left of this square with 3 dots and go to the little blue square shown in the map at left side.
You can go there 2 ways: via the south exit of this round building or via the manhole exit.
If you are at the 4 way tower; go right to walk the water-cliffside area until you get to the round building.
Enter the round building and: first see if you can exit to the south side.
If not, go to the top walkway of the round building to exit at top south.
Then walk to the attached ramp and be at the control room. Turn the panel so you can access the lower south exit.
My saved games are for green stand and last letter piece taken.
So it is before and way after what you need. If you want either one or both - tell me.