This has been covered extensively on the boards already. I am assuming you already have the necessary support packages; on linux you may need to install a C++ compiler and the zlib development library (these are called build-essentials and zlib1g-dev in ubuntu).
Untar your Rosetta package (tar -xvzf whateverit’scalled.tar.gz).
cd to rosetta_source
scons bin mode=release
Optionally, add -j# to scons.py bin mode=release, where j is the number of processes you have available with which to compile. On a 4-core machine, scons.py bin mode=release -j4.
It will take a while.
If you get “scons: done building targets” at the end, Rosetta is compiled and ready to use.