Hi Francis,
It looks like you've downloaded it from github, in which case you need to make sure you've renamed the folder so that, within your themes folder, you have a "decaf" folder (which contains a bunch of files and folders) - if the folder in your themes folder is called anything other than "decaf" (the zip from Github will call it "leizhang-moodle-theme_decaf-c8ff09c"), it won't work.
If you use the "extract to [zip name]" option that many unzip utils offer (as I often do), you might also have nested folders, as the zip contains a folder already - in this case, shifting the inner folder out into your "themes" folder and renaming to "decaf" (and removing the other folder if it's still there but empty) should do the trick.
Downloading from the new plugins database (http://moodle.org/plugins/view.php?plugin=theme_decaf) gets around the folder name issue, as the folder contained in that zip is already called "decaf" and therefore doesn't require renaming (though you can still end up with nested folders if you use "extract to [zip name]").
HTH,
Paul