Ok, first set some stuff up...(for the record I'm on OS X 10.3)
Click on the Apple Menu --> System Preferences --> Accounts and make a new user account (I'm assuming you have admin priv). I called mine "Mule".
Next, go to your Diablo Save folder (I'm not sure but I think all of your Diablo files should be in the Applications folder--mine are). Click on your Save folder and hit Command-I or File --> Get Info.
Go to ownership and permissions and go to details. Set everything to Access: Read and Write and then hit Apply to enclosed items.
(What you just did is allow the new user account you just made the ability to modify your diablo 2 saved files. You need to do this or else you'll end up accidentally duping items or having items disappear because they don't get saved.)
Everytime you create a new character, make sure to do that "Apply to enclosed items" step again.
Now, you start up a TCP/IP game on your main account. Then click on the top right corner (your username should be there). Select the new account you made and log in. Now start up diablo on the new account and join the game you made. You can now mule all you want to, and switch back and forth between accounts. Just make your you remember which account is hosting the game so you don't accidentally exit the game while you're muling, causing you to lose stuff.
If you've got more questions, email me at
[email protected].