To set up a Windows PPTP VPN server behind your OpenWRT enabled router, you need to forward the necessary ports and protocols, and install the proper modules.
Finding information about how to do this was a lengthy process. I never actually found the necessary info (although I noticed there are quite a few people which are rather clueless about networking; makes me wonder why they're bothering with OpenWRT in the first place, it's not the easiest firmware out there).
But I've managed to compile the bits and pieces of various sources into a coherent mass. It's not a guide in the literal sense, but will tell you want you need, assuming you do know your way around OpenWRT.