If VPN is running on the Mac and not the router I don't understand how that is going to sit outside of the firewall for the iPad to connect to. That's what I don't get. I asked how to do it and for instructions and no one has come through wtih them yet.
Unfortunately I don't have the time right now to type up instructions. Google 'iVPN' - download the app. It's pretty much self explanatory.
Then in your router you'll have to open the ports using port forwarding.
Then just type in the information that you input into iVPN (basically username and password) into the iPad's VPN settings.