Essentially you define a Kit by telling Creator which Qt Library to use (defined on the Qt Versions tab by selecting the qmake from the library bundle) and which compiler (Compilers tab) and some other bits and pieces. The compiler selected must be the same (actually binary compatible, essentially the same thing) as the one used to build the Qt library selected. These Kits are then available to select in your project's Project panel, which is also where you define the compile-time and run-time environments.