I found the manual for that model of Pioneer, but.. assuming you have indeed verified proper connection to the speaker(channel) in question, have you poked around the menu at all on this thing? It looks very straightforward - for example, you should be able to press RECEIVER on your remote, then press the SETUP button. From there you will see an option titled 'SP SETUP' - press ENTER. From there use up/down buttons to navigate to 'SP SET' which will open up the configuration in which your receiver is currently set. If the center channel is listed as 'No' or 'OFF' - change to 'Yes' or 'ON', however it lists it.
Also, another thing is to make sure your receiver is selected to process surround sound vs. multi-channel (which still should give you the center channel), stereo mode, enhanced stereo, direct, or any of those other modes that will not send a signal to the center channel.