Rosetta is a very useful program for the prediction of protein structure, folding and interactions be that docking with proteins or ligands.
However, the current version of Rosetta (3.2.1) uses SCons to compile itself on your system. This is usually fine except that these use gcc libraries 4.1 and before.
Ubunutu 11.04 considers anything before gcc v4.4 to be obsolete and you can't install them (even with apt-get).
As a result, a small amount of hacking is required.
First get SCons. Open a new terminal window.