First of all: welcome to the Asylum!

Now, to answer your question:
1. Select the profile for which you want to indicate the location.
2. Choose DVD / Personalize (Ctrl-P or the appropriate icon will work as well).
3. At the bottom left you will see the disc(s) included in the profile. To the right of this you can enter a location and slot for each disc.
4. Click OK and you're done!

Note: this will only work for profiles which have discs in them. If there are none, you'll have to enter them in the profile first. To do this:
1. Put the disc in your DVD-ROM drive.
2. In DVDP, choose DVD / Edit (or Ctrl-E or click the Edit icon).
3. In the window that opens, in the left column choose "Discs".
4. Click "Add Disc"
5. Change the description if necessary (default is Main Feature).
6. Click "Set".
7. Click Dual Layered if appropriate (you can verify this using e.g. Nero Info Tools - don't trust the DVD cover - they're often wrong).
8. Click OK, and you're done!

Note #2: if the profile is for a box set, please realize that DVDP works with so-called parent profiles and child profiles. The parent profile should NOT contain disc data (except when it's a TV show). In such a case, check the child profiles for disc info, and if there is none, enter it there. (If you have no child profiles in your collection, putting the disc in your DVD-ROM drive should trigger DVDP to recognize it - provided it's already in the database, of course).

Good luck and hope you'll enjoy the program!