Chad is right, backupsets are not supported for API data because the data is usually stored in a raw format that needs to be interpreted by the application in order to be usable. TDP for Domino happens to be the one TDP that backs up files from the filesystem, so you can get some of the data back from a backupset. But it is still not recommended or supported.
You may have noticed when restoring all the data, you get back files named similarly to dbname.nsf.data. This is the data portion of the backup, and the only piece that can actually be used from a backupset. You will need to request individual databases for restore using this naming convention. Once the file is restored, you should be able to rename it to dbname.nsf, and then open it in Domino.
It will not have any of the Domino meta-data with it, and may need to have fixup run against it in order to be opened, but it's the best you can get from a backupset.
Regards,
Stephen