The Screen blending mode uses the luminance (brightness) of a layer to make its changes. There is not enough contrast between your person and your background for screen mode to make a difference. Replace the green with white and use levels or curves on your man.
Your background needs to be white and your man fairly contrasty.
Screen and Add both add pixel values making everything lighter. Overlay makes light things lighter and dark things darker. Multiply makes dark things darker.
IOW, if you have black background and white text on a jpeg and you use the multiply mode everything will be black except where the white text is. If you use the screen mode all of the pixels under the black will not be changed but the pixels under the white will turn white. You'll have to experiment with the blend modes.
The tutorial uses footage of the guy shot against a perfectly white background. It looks like your footage does not. You'll have to remove the background or experiment with some other masking techniques and blend modes. Simple as that.
Ok, thank you very much guys!
I would do the following:
Create a white solid for a background.
Chroma key the green screen footage using keylight and duplicate they layer.
Use one of the keylight layers as a track matte for the forest footage.
Put the other keyliught layer at the top of the timeline.
Select a blend mode for this layer.
You should see the subject and the forest layer over a white background.