Sql Managment Studio will not let you directly manage or work on 6.5 databases.
After a search of support.microsoft.com it seems that you cannot upgrade the database. This is probably a good idea anyway as the schema of the databases changed and such.
If you want to manage the database, you need to reinstall Sql Server 6.5 and use those tools.
Alternatively if you want to convert the database from 6.5 to Sql2000, you still actually need to get a 6.5 system running first. Restore the database and use the upgrade wizard from a sql 2000 box.
If you just care about the data, you still should be able to bcp or query out the data.