Navigate to Service->Manage->Settings which can be found at the bottom of the page. Under PPP dropdown box, select "On" to enable PPP on your VPS and reboot your VPS.

To check if PPP is working correctly, enter this command over SSH (as root):
# /usr/sbin/pppd
If you see gibberish output for about 30 seconds and are returned back to the command prompt, PPP should be enabled on your VPS.