Atem Chromakey Problems

Having to use a system I've never touched before, but setting up a college multi-cam shoot on a green screen studio.

Followed all available instructions but have a camera pointed at the greenscreen (person in shot) and a digital background in the media player. when the two things are combined, it doesn't 'superimpose' the person on camera on top of the digital background, but tries to blend the two things together so that it's a blurred mess of background, person and green screen.

Apologies for description, I'm not a technician, but have so far followed what I thought were all the correct procedures and don't understand why the pictures are blending together in a real messy image.

Can you post an image?

Start by putting the media player background image on air.
Go to the keyer section in the software and make sure that the keyer that you want to use are set to chroma key.
Set the chroma keyer to the input that you have the camera pointing to the green screen on.
Set the chroma keyer to match the green background.
Put the keyer on and adjust the setting to key out the green background.
The amount of adjustments you can make depends of the type of Atem you have.

The television studio switchers manual have a good description of how to do a chroma key on page 130.