I am not very sure my answer is correct. But here goes.
Look at Page 6 of your manual. There is a set of inputs that are labelled AUX and closed with a cover. If you open this, you will find various inputs jacks such as optical, SVideo, L&R. etc. One of these have to be used for your mike. You may also to go to setup and set the input point you to use and set it for the resistance and other factors needed for the mike.
I am not even sure your AVR can be used as a Karaoke. Though it says somewhere you can mix two inputs, these are audio and video from two different sources.
As I said, I am not sure my answer is correct. Check with a Marantz dealer or send an email to their support center.