I've never used that model, though I have used a tricaster. They treat chroma keys as different different channels entirely. For example, if channel 1 is green screen footage and channel 2 is background footage, channel 3 would be they chroma key that combined the two and channel four would be something else entirely. You essentially have to tell the chroma channel to use channels 1 & 2. If you switch something else into channel 1, channel 3 also changes.
I don't know if that's how your mixer works or not. All I can suggest without looking at it is to consult your manual. :/
