I'm not familiar with how Samsung's task manager interface works, but you could try a couple other methods:
1. Go to the App Manager and look for the TouchWiz homescreen or launcher app. I'm not sure what it's called, so perhaps someone else with Samsung devices can chime in. It might be called "Homescreen" or "Launcher" or "TouchWiz." Anyway, once you find it, select it and Clear Defaults. Now press the Home button--if you have Nova installed, it should prompt you to choose Nova or TouchWiz as the default whenever you press Home.
2. If you're not sure which app is the TouchWiz launcher, you could always Clear All App Preferences, which usually comes up if you press the Menu button within the App Manager. Keep in mind that clearing all defaults will also re-enable any disabled apps.