I know Tape Library configuration can be tricky when working with DPM. If you are using a Dell TL 2000, make sure to use IBM driver. Here is the procedure I always use:
1.Download the latest driver from the IBM site -ftp://ftp.software.ibm.com/storage/devdrvr/Windows/
2.Uninstall any previously installed drivers for the drive and library.
3.Reboot the server.
4.Extract the downloaded driver to a folder.
5.Run install_nonexclusive.exe -t -d command and accept the non-windows certified driver warning.
6.Run install_nonexclusive.exe -c -d command and accept the non-windows certified driver warning.
7.Reboot the server.
8.Perform a rescan from DPM admin console.
You should be good to go...