Can you be a little more specific on exactly what you're trying to do?
Do you want to add to your present library and are you talking Media Center specifically.
If you're doing what I'm guessing, in Media Center go to Tools>Options>Library & Folders
Under Auto-Import there is a small box Configure auto-import
Left click one time on that title and a box will pop up that lets you add folders where you would like the program to watch. These folders will become your library. If you're using a complete drive, just use the drive letter (i.e. G:\ for the "G" drive)
I'd suggest the following in the bottom box for checkmark selection:
Analyze audio for audio files
Ignore files previously removed from library
Update for external changes
Fix Broken Links: Yes (protect files on missing drives)
Write file tags when analyzing audio....
These may already be checked, but it will save you work later. You can check the others if you prefer.
Then click "Finish" and the program will start adding your files.