How to find mac address?

It depends - is the computer on the same networt segment as you are, or is there a router between you and that computer? If a router is involved you're out of luck, because MAC addresses are not surviving router traversal. Otherwise you could open a command prompt and use "arp -a" to find the IP address as well as the MAC address next to it. Keep in mind that if you're going across a router that arp command will work as well, but it will show you the router MAC, not the destination MAC.