Well, it doesn't seem to be in the software repositories, but I did find a request for someone to package it up. Maybe it will be some day. It doesn't even seem to need compiling.
On a command line,
Code:
wget "http://www.celtx.com/download/Celtx.tar.gz"
tar xvzf Celtx.tar.gz && cd celtx
./celtx