fce 4 chroma key problems

I have been using fce 4 for 2 years now without too many problems. Now that I have tried to edit green screen footage, I am having problems with chroma key and colour corrector. Basically, I follow the instructions as per Tom Wolsky's book, but the effect is not 'applied'. It seems like the tools are there but not connected somehow.

Can anyone help, please? It's on a G5 10.4.11 2 GHz, 2GB SDRAM and the program is 4.0.1