Copy your POT files (I assume you're talking about templates, not just
backgrounds) to this folder (Windows XP, it should be someplace similar for
Vista):
Documents and Settings\UserName\Application Data\Microsoft\Templates.
When you go to File | New and choose My Templates, the POT and POTX files
there will show up in the list.
To make them show up on the gallery on the Design ribbon is harder. This is
probably by design, otherwise the Theme gallery would become very cluttered
very fast.
Basically, you'd apply the template to a presentation -- I'd do it as
explained above: put the POT files in the appropriate folder, then File |
New and choose My Templates.
Then go to the Design ribbon, click the dropdown arrow in the Themes
gallery, and choose Save Current Theme. After this, a new "Custom" category
will be created on the Theme gallery and the POT file (which has now been
saved as a Theme, THMX extension) will show up above where it says
"Built-in."
I have a few there I don't remember specifically saving, so it may be that
after you start presentations based on specific themes or POTX files, after
awhile they automatically show up in your "custom" area on the Theme
gallery. I don't know for sure, though, and I don't know if POT files would
do the same or not. I think so, but again, I'm just not sure.