Alright, the issue- My brother and I bought the game in order to play together online, now when you run Multiplayer it says "Use the Multiplayer Key Changer used under the start menu". Where and how do I get to this? Is multiplayer even available anymore?

read the cd-key threads?

In the Neverwinter Nights 2 Complete folder after you install, look for "SerialTool.exe" run it, it will ask for your CD Keys, which will be in your account info for your game on this site.

Multiplayer is still available because fans/users run the servers rather than a company, but the gamespy server that listed the available game servers is not. nightshift5's instructions are correct for changing your keys. To see a list of online servers you will want to do that, then run the updater (nwupdate.exe) in that same folder, then install the NWN2 Client Extender. Make sure you and your brother agree on the same server in order to play together. I have some more information in my post here.