How to enable VPN clients to access LAN computers that are not running VPN software

I installed an OpenVPN service on our server. The default setup worked right out of the box. However, I wanted to use a VPN client to perform offsite backups of the computers in our LAN (using the backuppc software). First I installed OpenVPN clients on all of the LAN computers to be backed up, but OpenVPN on MacOS X seems to be a bit unreliable. Therefore I wanted to setup the VPN server to forward requests to our LAN network. The following changes were necessary: