I wonder why I need to enter the keycode everytime I need to access the multiplayer, don't matter if I choose to save my info or not, or if I choose modify instead of typing in the code when I get asked for it.

Also I, as alot of others also get "keycode already in use" when I try to play online :/

I had this problem as well, and I do believe I have solved this issue myself.
When you FIRST start the game, at the login dialog box, towards the bottom, there should be something that looks like a link to a website that says "Modify" This should bring up another dialog box that asks for your CD Key again, just enter again, and it should save the key permanently and shouldn't "forget."

Read the sticky at the top of the forums......