Backup tapes work a little bit differently than your average read-write media . They typically contain proprietary information regarding their format and the data. You usually need to use a program that can understand this format to recover the data -- rather than just "read" from the tape as you might a CDRom or Harddrive with Windows Explorer.
To recover the files contained within your backup set from the tape, you will need to use NTBackup (or Veritas BackupExec, which can read NTBackup files). You will not directly be able to restore from tape to a BKF format that you may be familiar with NTBackup using for backup to disk scenarios.
The data that is written to tape actually is stored in Microsoft Tape Format (MTF), so any software that can read MTF can recover your data.