It should be as simple as adding the following commands:
> aaa-server PDMusers protocol radius (or tacacs+)
> aaa authentication http console PDMusers
> aaa-server PDMusers host x.x.x.x
Or if you already have the two "aaa-server" commands in your config (for your PPTP sessions), then all you'd need is the middle command.
What does the log on the Radius server show when you try and authenticate? What does PDM show exactly when you try and access it? If you enable logging on the PIX with:
> logging on
> logging buffer debug
> sho logging
what appears in the syslog after you try and authenticate?