The white button is the magazine safety plunger and isn't involved in this process.
It's been awhile, but IIRC, locking the firing pin forward with the safety plunger doesn't get the firing pin clear of the manual safety. You need to push-and hold- the firing pin forward so that it's clear of the safety body and then gently remove the safety. BEWARE- there is a spring loaded plunger in the left hand side of the safety body that's going to come free as the safety body clears the slide. DO NOT LOSE THESE PARTS! BTW, don't confuse the plunger & spring from the end of the safety barrel that retains the right side safety lever with those that are internal to the barrel and provide the safety detents. They aren't interchangable.
