http://www.rdpslides.com/pptfaq/FAQ00080.htm has a list of links to the
templates at MS. Shimmer doesn't seem to be one of the available
templates. I'm guessing that it's because that's a new template released
with PPT 2002. The template collections at MS say "This download is a
collection of PowerPoint design templates from PowerPoint 4.0,
PowerPoint 95, PowerPoint 97, and PowerPoint 2000." The other templates
available at MS are new ones which weren't released in conjunction with
PPT.
You can use an existing presentation based on shimmer.pot as a template,
though. When you go to Format/Apply Design, you can choose a regular
presentation instead of a POT file.
In PPT 2002/2003, you'd do this from the task pane where it says "new
presentation...from existing presentation."
(Helps if you tell us which version of PPT you're using. Then again, I
suppose if you were using 2002/2003, you'd already have this template on
said system.)
Or you can create your own version of the shimmer template. Just open a
presentation based on it, delete all the slides, and File/Save As.
Select Template (*.POT) from the "as type" dropdown.