I used this guide from http://rawtherapee.com/downloads/:
Repository 1 - PPA
This repository is kindly run by Dariusz Duma, and all questions should be directed to him.
To install RawTherapee follow these simple steps:
Open the terminal and paste this code into it:
sudo add-apt-repository ppa:dhor/myway
sudo apt-get update
sudo apt-get install rawtherapee
If you want a development version:
sudo apt-get install rawtherapee-unstable
RawTherapee should now be installed and ready for use.
EDIT: Just installed the unstable version for testing and its working.