I have problems with connection to DD-WRT router with PPTP server installed. Win7 has no problems with connection to it. But in Ubuntu I'm getting all the time:
Dec 28 21:52:14 IT-nb pppd[4209]: Plugin /usr/lib/pppd/2.4.5//nm-pptp-pppd-plugin.so loaded.
Dec 28 21:52:14 IT-nb pppd[4209]: pppd 2.4.5 started by root, uid 0
Dec 28 21:52:14 IT-nb pppd[4209]: Using interface ppp0
Dec 28 21:52:14 IT-nb pppd[4209]: Connect: ppp0 <--> /dev/pts/2
Dec 28 21:52:18 IT-nb pppd[4209]: Connection terminated.
Dec 28 21:52:18 IT-nb pppd[4209]: Exit.
Whereas Windows appears to try different options automatically, you must configure Network Manager more manually.
In Advanced Options there are a number of options; you need to find a combination that works.
You can try also Kvpnc, I used it to connect via PPTP and it worked. You can install Kvpnc directly from the Ubuntu Software Center:
https://apps.ubuntu.com/cat/applications/maverick/kvpnc/
You can use the guide here, there are some screenshot on how to configure your PPTP connection:
http://vimalkumar.in/2008/09/19/howto-connect-to-a-pptp-vpn-server-using-kvpnc/