Your real problem here is a disturbing situation where you're apparently defaulting to permissive access. My gut instinct is to first question if this is not bogus. Can you transmit data over the established PPTP connection after authing with no username and password? If not, then what is the evidence that the server has accepted the connection?
Then, go get yourself some visibility. Read how to log and/or debug and then watch the log file. Debugging is a lot harder when you're running blind (but you'll be making the Windows community proud).
And find a real HOWTO, not just some pox snippet that did well in the SERP.
Google for Linux PPP HOWTO and Linux PPTP HOWTO or look for the README in your distro.
When you find a real HOWTO, try and check the versions of PPP and PPTP you're using as there's a bit of a history of version gotchas. Then learn the config file syntax and experiment. This is essentially the path the README/HOWTO takes you down.