Hello, he have to configure the routing of your audio system (ASIO? MME? Direct Sound? Windows Soundmapper?) so that Music Studio can record the output of a parallel working application. If you don´t have a special audio system check your windows audio system (soundmapper) and select "stereo mix" as input/recording device - now it should be possible to select that in Music Studio.
Otherwise it will be better to simply bounce your Fruity Loops Project (Export Options) and load these tracks or the stereo bounce into music studio.
The 3rd way: use Fruity Loops as Rewire Client and activate Music Studio as Rewire Master (in VST Menu) - now you can use Fruity Loops like a plugin inside Music Studio - please refer for the Help menu of Fruity Loops regarding that procedure.