Where do I connect earth wire from turntable ?

There's nothing special about a ground wire! All you need is a piece of single standard wire that connects to an earth point on either the receiver - generally a screw terminal, or a earth point in your home. Say a metal pipe. Any electrical shop will have a strand of wire that will do the job.

A grounding wire is just a piece of wire connected to an earth point on the deck, connecting to an earth point say on a radiator pipe. There's nothing special about this wire any old piece will do. You only need it if you get a hum noise when the turntable is connected to the amp.

It could be a bad connection of the earth pins to the cartridge. One turntable I had hummed even with an earth connection. I found out that the cause was the connection of the earth wire in a 3 pin plug.

The noise will be earth feedback. You might need to connect a wire to an earth point on the turntable - that goes to earth (a metal pipe etc). Sometimes the earth feedback is caused by the earth cable (green/yellow) in the plug either connected or not!

Earthing a turntable.
There are 2 common ways of earthing turntables that have an earth wire as a standard fitting.
If the amp has dedicated phono input there will also be an earth terminal post nearby.
For amps without an earth terminal simply slip the earth wire into the side of one of the turntables rca output connections as you plug into the amp.

Cheers John.

yes you can but you will need a phono preamp such as the one found here.
http://www.turntableneedles.com/Phono-Pre-Amps_c_4.html
Hook the output of the preamp to Line 1 on the back of the CR515 and for simplest ground wire hookup, choose a preamp with a thumbscrew for the ground (earth) cable coming from your turntable.

There should be a connection point on the turntable to connect an earth wire. Simply connect it to a metal object that is earthed, some amps have an earth terminal. Sometimes if the mains plug on the deck has an earth lead on it, it can cuase a hum. On the other hand sometimes it needs connecting.

Fasten the ground to one of the metal screws that hold the metal(?) cover on.You want to connect it to the metal frame of Onkyo.

Yes first check is that the Gnd wire from your turntable is connected to the Gnd (ground) post at the back of your amplifier with a good clean tight connection.
If the problem persists, check ALL your hi-fi components mains plugs for earth connections including your turntable, then remove the earth connections on all but one component so that you are left with only item earthed.
What does sometimes cause problems is if 2 or more hi-fi separates are earthed and connected to each other the earth cables along with the hi-fi interconnects form an inductive loop that picks up interferance such as mains hum etc.
Regards John