While I'm running Fritzing on Windows, since none of the Unix folks seem to be replying I'll have a bash 
it sounds like you have permissions problems somewhere. Try an
ls -l /usr/share/fritzing/sketches/core/Button.fzz
and an
ls -l ./
if the user ids and permissions are different then that's likely your problem. One reason for this is if you installed Fritzing as one user (such as root if you used sudo to install) and are running as another user then I expect (but don't know for sure ) that you may hit this problem. There have been a number of posts lately about Unix install issues and I think a couple of pointers to install examples so you may also try a search in the forums for either Ubunto or Linux and see what turns up.