PgAdmin doesn't (directly) support Restore'ing from plain text backups. To restore a backup, backed up using PLAIN format, you'd need to pass it through a command line utility called 'psql' that is available in both PgAdmin as well as postgres server installations.
I think it's high time that PgAdmin support this feature, especially when all the building blocks are already in-place. Lets see if I could take out time and submit a patch for this.
... till later.