Go Launcher for Galaxy S2

Hi guys
every time i finish using an application it throws me back to the Android OS but ive downloaded a 3rd party thing called Go Launcher EX how do i stop it throwing me back to Android OS i want it to go to go launcher ex as default but i aint getting the option to do this.
Answer #1
i have gingerbread on my desire, so not sure if it is the same way on ics (if you have it):
Settings>Applications>Manage Applications>All>
Then find the default launcher (Launcher for me), and tap “Clear Defaults” to well, clear defaults. now when you press/tap the home button you’ll have the option to select between launchers.