If your trying to login to the domain over the internet, I have done this with SME. You have to set your DNS (& WINS if used) to the appropriate Windows server IP address in advanced Network settings for the VPN connection.
Windows 2k and XP Pro, has a check box on the login box to make a dial up connection prior to logging in. If DNS is working, your computer will be able authenticate against AD.
I would not depend on SME 5.6 PPTP. Regardless of what Mitel says, PPTP vpn is unstable/not fixed even with U4 applied.
good luck,
ryan