Welcome to Ubuntu! Which version are you using? Setting up routing under Hardy was quite easy, but setting it up under Intrepid is not that difficult either.
I'm no expert in VPN. However, what I do know is that you need to know the range of IP addresses which must be routed through the VPN. In other words, you need to tell Ubuntu that whenever a request is made to contact an IP is the specified range, it must route that request through the VPN.
In Hardy this can be setup via the VPN plugin (on the last tab I think). In Jaunty, you need to follow the instructions in the second part of my first post.
If your corporate network uses addresses in the range, for example, of X.X.1.1 to X.X.255.255 then you would use the expression X.X.0.0/16 to specify all those addresses. If you use X.X.X.1 to X.X.X.255 then use X.X.X.0/8. You can see that the first one has more addresses in it than the second one, because the first one only has two X's and the second one has three X's.
All of this applies to my experience using PPTP VPN. There are other systems like Cisco VPN and OpenVPN, but I don't know anything about them or how similar they are to PPTP.
Good luck. I can also suggest searching on the 'net for tutorials or assistance with this. I'm sure you'll find something that will help you.