Hi,
what worked for me was navigating to /usr/bin or even /bin within the
"Choose Helper Application" dialogue and then selecting an executable
(in this case libreoffice). The "Choose Helper Application" dialogue
will even remember it was in /usr/bin when you invoke it again.
Reminds me of the way to do stuff in Windows Vista - back when I was
young and ignorant ;) - where you'd "Open with" something like
"C:\Program Files\Microsoft\Microsoft Office\WINWORD.EXE". Not quite an
elegant way of managing programms, but it works most of the time.
Regards