Step 1: good. Steps 2 and 3: Why?

After step 1 the Qt SDK was fully installed and functional. If you installed only the Qt Desktop libraries for MSCV2008 then only those compiler options will be available to build. Install the MingW libraries using the package manager if you have not already. If you have installed both the MSVC libs and the MingW libs and still cannot see the MingW compiler options then visit Tools Options in Qt Creator and ensure that the MingW Qt location is listed on the Qt4 panel.