If you insist on TeXStudio you may now download TeXStudio 2.4. You can now set up your
lualatex commands with this version.
I am using Ubuntu 12.04 but TeXStudio 2.4 must have the same features as the Windows version.
See in the picture below that there is already a separate box your
Now, in the
build option, choose your default compiler. You may now choose either
lualatex. See picture below.
If you don't want to update just yet, you may look at my comment in Kurt's answer.
But it is a hassle to keep on changing your default compiler. Some times it is better to just open
cmd and run
xelatex myfile.tex or
lualatex myfile.tex. Just CD to your source folder before doing the compile.