I recently purchased a new iMac running OS X 10.5 for my home office. I'm running Parallels so I can develop code in Visual Studio.net and SQL, and graphic design work we're now doing in the Mac environment. I ran into an issue when I wanted to connect to my office router, a linksys RV082. In Windows you need the Linksys Quick VPN utility to connect to the VPN, but Linksys provides no such utility for the mac OS. Plus, the QuickVPN utility crashes when you try to run it on Windows Server 2003 running inside of parallels. Since I couldn't find any documentation online for how to connect to the VPN from a mac, and no forum posts I could find got it going for me, I had to do some trial and error testing. I finally got it working so I thought I would post the solution here for future reference.
1. In the Router admin console, go to VPN >> PPTP Server
2. Create a user account and password and make sure that the PPTP server is enabled. Save Changes.
3. On your Mac, open the network preferences dialog. Click the + icon under the list of all your network connections on the left to add a new one.
4. Set Interface to VPN, then select PPTP from the VPN Type dropdown. Name your service and click Create.
5. Enter the server address (public IP of your router) and the account name you created. Encryption set to automatic worked for me.
6. Open Authentication settings and enter your password.
7. Click Apply, then connect.