To find out your network card's hardware (MAC) address, find your operating system below and follow the appropriate instructions. You will need to supply your MAC address when registering for a LAN party.
The MAC address will be a series of 12 characters, grouped in twos: for example 00:26:54:11:5E:95 or 00-06-5B-C7-80-9F.
Press the Start button, go to All Programs, then Accessories, then click on Command Prompt. A black window with some white writing should appear. Type "ipconfig /all" and press Enter. A lot of text should appear. Near the top, under the heading "Ethernet Adaptor" (or something similar) there should be a line that says "Physical Address". The 12 numbers/letters next to it is your MAC address.
Click on the start menu. Choose Run. Type "cmd" in the text bar and click on OK. Type "ipconfig /all" and press Enter. Your MAC address will be the field labelled "Physical Address".
Run "/sbin/ifconfig -a". If you have one network card then the HWaddr of eth0 should be your MAC address. If you have more than one network card then you'll have to work out which one you will be using.
If you're running something not in the above list, you will probably have difficulty running Steam; whilst it may be possible, you're on your own. If you do manage to make it work, you're welcome to participate.
Acknowledgement: these instructions were borrowed from the Trinity Hall Computer Support Group.