I've noticed you're a Linux newbie - welcome!

When you install acrobat by default, I think it sticks in in
/usr/local/Acrobat5
(depending on version). What you can do quickly, rather than adding a specific location to your path for one measly app, is to setup a link in a common path local - like /usr/local/bin. As root:
cd /usr/local/bin
ln -s /usr/local/Acrobat5/bin/acroread .
(note the period - it's critical). Then, start a new shell and launch Houdini - it should be in your path. Obviously double-check that the location I've suggested is accurate.
Cheers,
J.C.