To find the built in Toon EFX in Carrara 5 you need to click on the SCENE tab, click on FILTERS, Click on the "+"-- then you should see a dialog box for TOON EFX for "Toon III." ...

Make sure you are in Realistic rendering mode as if NPR is on it won't work. The Toon EFX are a POST effect that happens after Carrara completes the ray-traced render. The Toon III dialog gives you a small render box to get an idea of what the toon efx will look like...