Hi Oasis!
The Library files with a *.cat extension are meant to be opened with the Little Reader software. The BrillKids Library has dedicated sections for each BrillKids software (Little Reader, Little Math and Little Musician), and any file that's under one of the three BrillKids software categories will only work with the corresponding software.
In case you haven't done a Little Reader free trial yet, you may want to check it out so you can try the *.cat files you dowloaded in the trial software. You may refer to this article for instructions on how to get the Little Reader free trial:
http://support.brillkids.com/entries/20390567-How-to-get-a-free-trial-for-Little-ReaderIf you are looking for lesson or activity sheet files that are in non-BrillKids software format, we do have free-format files under the "Free Downloads" section. Most of the files there are in pdf or ppt formats.