How do I get this rusted screw out?
Some times tightening the screw helps un stick it.I don't mean turning it a full quarter tighten but use some force. If you hear it click/budge a bit- Spray WD-40 on it then untighten it, spray WD-40(only a short spray-don't drown it), repeat over and over and the distance will get larger and larger and eventually you will be able to unscrew with ease.


By the way- Do not use a flat screwdriver. Get a spanner that fits snug on that bolt - you will get it loose very quick with that. 
To avoid this problem in the future- spend a few bux more and get stainless steel bolts/screws. Galvanized are OK- but some cheap ones rust anyway after a year or two.