1. What can dark matter be used for?
100 Dark Matter lets you place a Dark Matter Energy Turret (yet to be completed).
2. In Gunreal players cannot double jump and the Quadjump mod doesnt work. Any plans to allow double jump?
Nope. Crouch-jump allows you to reach all double-jump areas in maps.
Double-jumping while in combat is a non-Gunreal way of doing things.
3. You can only hold 2 weapons?
Usually 3, unless you are carrying multiple large, 4-slot weapons.
4. When you have 2 weapons, how do you switch one weapon for another (on ground)?
Middle-mouse click. Might want to watch the "how to test" video, which answers most of these (and more):
http://www.gunreal.com/download.htm