That screw is not used to add coolant. It is the coolant bleed screw.
Here's the proper way. (or at least how I do it -- no problems) Drain entire coolant system.. radiator and block. Mix coolant and water together. Remove coolant bleed screw. Pour coolant/water mixture into expansion tank (it will naturally flow into block with screw open) until coolant begins to come out of the bleed hole. Screw bleed screw back in. (it will take about a gallon and a 1/4 before the coolant reachs the bleed screw. Pour the rest into expansion tank until full. Run car with heater on for a few minutes. Turn off car and wait about 15 minutes for coolant to settle back in. Add more coolant if necessary.
I suppose you could add coolant via that bleed screw but there's no guarantee that the coolant will stay in block... it may travel toward the radiator.
**edit** Just saw Mike Richards post above. If this is the way the CD explains it, then it's the proper way. The '86 paper shop manual explains it the way I do it. I suppose either way is fine.
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi