I have the TX-SR393, their entry level model and I've had the same issues.
For settings, try pressing the settings menu on the Onkyo remote and when it shows on the receiver display, then switch to the receiver/arc input on the TV itself. The problem is that your receiver isn't sending any video by default, which is why it's not being detected. But if you switch to the full screen settings menu on the receiver, it should then be detectable because it's sending the menu. (But the quick menu will not be as it piggybacks on whatever else is showing.) You may or may not need to do this before sending ARC content to the receiver. I can't remember. But I have found it's fussy about switching to and from the TV.
Unless you are sending a video signal through the receiver, you're not going to be able to see the info/mode information on the TV. You just have to look at the front of the receiver, which isn't nearly as detailed. But it should be enough to at least sort out two channel and 5.1 modes.
