How to enable X port forwarding with ssh

[Juraj Fedel <w4m@inmail.sk>]

While using ssh on local network (actualy qemu virtual machine)
I have hard time starting X application on virtual machine and
let it show window on host X window. Starting

ssh -X

does not help, nor does following content of ~/.ssh/config on local
computer make any difference:

Host *
	ForwardX11 yes

In both cases I can see this message when passing -v option
debug1: Requesting X11 forwarding with authentication spoofing.
and when I try to start xclock I get error:

Error: Can't open display:

So how do I enable X port forwarding?
Juraj