i am told that underexposition cannot be corrected as much as overexposition. for the latter part, see for example
this.
anyhows, this is not a gfx forum, it is more of a geek and (point & shoot-) photography forum.
recovering under- or overexposed raw shots is "an art" for itself, and a thousand different people use 2000 different techniques & tools to do that. anyhows, i think it can be done with most programs that support either the raw files or the dng files, there are a lot of free tools available, check for example "raw therapee". like bugmenot said, work with the sliders, read tutorials about raw editing & the rest will follow
