Do you have VSS 2005 installed? You need it rather than the previous version (6.0d).
Also, do you really need the history in TFS? Or can you draw a line in the sand and say that all history before such and such a date is in VSS and all history after that date is in TFS? If so, you can simply do a get latest from VSS and add the files into TFS. Migrating is non-trivial because you need to deal with VSS users that don't map to domain users, VSS users that don't exist anymore, and although the order of source-control operations is maintained the actual date/time of the operation isn't migrated, it is however stored in the comment as part of the migration.