First, can you ping the other end of the tunnel? If you don't know what it is, type the command "ifconfig" and look for the entry for tun0 or tap0. You'll see the IP address of the tunnel on your end, and the IP address of the other end of the tunnel as "P-t-P" ("point-to-point"). The entry should look like this:
This indicates my local machine has address 10.1.1.20 and connects over the tunnel to 10.1.1.1. So to test the tunnel, I'd run "ping 10.1.1.1". If it times out, the tunnel is not configured correctly.
inet addr:10.1.1.20 P-t-P:10.1.1.1 Mask:255.255.255.255
If ping works, then run the command "route -n" and see where the default gateway points (the route for Destination 0.0.0.0). Does it use the other end of the tunnel?
Preserving Internet connectivity on a machine using a VPN can be tricky. You have to maintain a direct connection to the Internet so the tunnel can be constructed, but then send all the appropriate traffic over the tunnel.