I restored my library, but my playlists are empty since they point to files in their original location on the C: drive.
That shouldn't be. Did you do File > Library > Restore library and use a backup from your other computer? If so, you should have all of your playlists intact. None of them will play because they can't find the files on the C: drive. But they should be there; just with red minus signs on them, indicating "can't find file".
You might try turning off the fix broken links option just in case it's part of the problem:
Tools > Options > Library & Folders > configure auto import > tasks > fix broken links > No
Where can I edit my playlists to change the file path so that the playlists repopulate?
Once you import your library backup from your other computer, all of the files will point to C: as you have indicated. You can then use the Rename, Move, and Copy files tool to change their paths from C:\ to D:\ using the Find & Replace mode of the RM&C tool.
See the examples on this page for some help getting started:
http://wiki.jriver.com/index.php/Find_%26_Replace_TemplateIf you need more help, just ask. RM&C is a wonderful tool, but it can be a tiny bit confusing the first few times you use it.
For your task, you want to use UPDATE ONLY mode, and the Find & Replace template only. Uncheck Directories and Filename; they aren't needed for this particular task.
Good luck!
Brian.