Hi
Open the explorer view (press 8)and change to current pass (press U). Then click on the icon on the default pass and a property page will open..
On the default pass part of the property page click on the OUTPUT SHADERS and then press the ADD button..
In the window that will open go to shaders>output (if it's not already there) and select "2D_background_color" if u want a color, or "2D_background_pic" if u want an image as background..
Then back to the property page and the output shader stack select the shader u have chosen and click the INSPECT button to adjust the settings (color, image ect..)
Best regards
George