Are you using the HDMI out on the receiver to go to the TV?
If so, as stated in the manual for the receiver, you can't output any other input signal to HDMI other than an HDMI input.
Therefore, composite, s-video, and component inputs can not be sent to the TV via HDMI.
Therefore, if you're running the Wii with the included composite cable, then you may have to use the composite out on the receiver to the TV as the receiver may not be able to convert it to another output.When a non-standard video signal from a game machine or some other source is input, the video conversion function might not operate
Also note that if you are using the component cable with the Wii, then you must use the component out on the receiver to the TV.