Just a thought, but did you download the MPlayer software when you installed?
I played many hours on multiplayer. What I did was go online first to MPlayers home page, select BGII from the list of on-line games and it downloads some updates then takes you into a forum where you can choose to join or create a game. You will need the BGII CD in your drive (Disk 2 if I remember correctly!)
If you are choosing multiplayer in offline mode, ie to be able to create a team of six characters of your own choice, then I am not sure what is going wrong for you.
To create/join an on line game you need to create your main character then export it. Go online to MPlayers home page etc (as described above - enter BGII forum) then choose to join or create a game. If you create you will become "modifier" see your manual for further instructions. Then when you go into the load screen you assign the character slots to whoever is joining you in your game. Here you assign yourself a character slot and import your exported character. Remember to keep a couple of slots free if you intend to take on Imoen (party of the storyline!), Jaheira and Minsc. These three can be assigned to players after they have joined your team.
A point to remember if you create a game is that if you do not have any limitations (ie maximum no of players, passwords etc) you will get people dropping in and out of your game without joining and this really slows things up. And your PC spec is fine. Before I upgraded to a ferrari of a machine I was playing online perfectly ok with a 233, 64Mb ram and 8Mb card. However, one thing to remember is in the BGII config, if you have made any adjustments to the frame rate under AI options you will need to reset this as all multiplayers will need to be on the same frame rate (default is 30 I think)
Hope this helps, good luck (on-line gaming is cool! - your characters really DO have a personality of their own!!)