sorry, having read your post again I suspect it's even simpler then that. Try selecting the .mint file, then pressed enter. If this works, it's because your moving the mouse too much when double clicking.
Pierre: this is to do with .mint files and mintInstall, not the ubiquity install on the liveCD
EDIT: it seems this is a bug in mintInstall afterall, look out for an update soon.
If you have a question that has been answered and solved, then please edit your original post and put a [SOLVED] at the end of your subject header
Hint - use a google search including the search term site:forums.linuxmint.com