Unlike most phones that support PPTP but not OpenVPN the Maemo OS based phones do support OpenVPN. Therefore this is the best choice for a VPN on a Nokia N900 or similar phone.
1) Download OpenVPN for Maemo here
2) Then download the OpenVPN GUI here
3) Download the SurfBouncer VPN configuration files to your desktop in a folder with a name such as 'SurfBouncer'
4) Upload the configuration folder to your N900 via your Bluetooth connection from the PC
5) Open the OpenVPN applet and press 'manage connections' and then ' new connection'
6) Load a file that ends with '.ovpn' into Configuration File
7) Load the file that ends with '.crt' into CA file
8) Load the '.key' file into Key File
9) Import the configuration
10) Repeat this for all the various .ovpn files you have in the configuration folder you downloaded
11) Connect from the OpenVPN GUI applet menu