installing with a manual portfile

Hi all - I realize that a lot of documentation was lost with the move. I've had a hard time finding an answer to my question, but please point me at the docs if they do exist. I'm trying to get GnuCash running. I found a bug on Trac that says it needs an older version of slib. I have the Portfile for the older version, and I've deactivated the newer version. How do I install from a Portfile that I downloaded from Trac that's not a part of the regular source tree? I tried port install -D <path to new portfile> But that didn't work. I could edit the Portfile in the normal port tree, but I'd rather not muck with that so it stays up-to-date. Thanks, Michael