I also tried POPTOP, and ran into the same problems as you. I gave up, and I am now using OPENVPN package. It works very well, and I can highly recommend it.
I want to use wl-500gx as a VPN-server. It seems, only poptop package available. Is it possible get it running? I spent some time to tune it, but no way.
I have found only http://wl500g.info/showthread.php?t=3032 where desribed some steps. poptop runs, but it is impossible to connect to it. When i use MSCHAPv2 poptop can not authenticate etc.
I also tried POPTOP, and ran into the same problems as you. I gave up, and I am now using OPENVPN package. It works very well, and I can highly recommend it.