If these are WinRunner, QTP or VUgen scripts, then you can do the following:
1. Open script in testing tool where it was developed
2. Connect to TestDirector
3. Click "Save As..." and select "TestDirector" mode (when you are connected to TestDirector, you can save scripts either to file system or to TestDirector tree)
4. Save script to any folder you want in TD.
If you have a huge number of these scripts, I think "Import Tests" utility is available on support site.
If you want to import manual tests, written in Word or Excel, use Word or Excel plug-in (again, can be downloaded from the support site).
For other script types: it depends. If you have any other type of scripts, then need more details.