I'm running a PPTP VPN server on my DD-WRT enabled router, and it works inside the network but obviously not outside because I haven't opened and ports. What ports need to be open for the VPN server to accept outside connections?
The PPTP server in dd-wrt does not need to have any port-forwarding rules, or firewall ports opened specifically for it. Enabling the server to run will open the appropriate ports facing the Internet.
Read through the setup guide for instructions on how to configure your client properly.