I am moving this post fromthe sharing and storage forum to the premiere elements forum.
They should be able to answer your question there.
Well, there are three groups of Instant Movie Themes.
The first is a very limited subset, installed with the program. I do not know the count, but I would expect maybe 1 - 3.
Then, there is a additional subset, which is much larger, and installed with the Additional Functional Content, which contains DVD/BD Menus, Titles, etc. (in addition to the few installed with the program). This is where the bulk of the Themes, etc. are. It depends on whether one installed from physical media, or from a download file. With the physical, it is a separate installation, done after the program has been installed, and on a separate disc, in most cases. With a download, it is a separate download file. Did you install from physical media, or a download.
Last, with a Photoshop.com Plus membership, from time to time, Adobe made a very few extra Themes available for download, but not that many, and a lot of subscribers were dissatisfied with that extra content. Though I have never seen the Plus material, it seemed that Adobe would offer maybe one holiday-oriented Theme per Season, i.e. a Halloween Theme in October, etc.
Please let us know a bit more,