How can I figure out my MAC address on my Mac OS X system?

Great job wanting to keep your network secure, I applaud that. In fact, it's amazing how many people have completely open, public networks which puts them at significant risk of getting in trouble with the law. What kind of trouble? Well, if someone taps into your network and hacks into a bank, for example, or downloads lots of movies, you could find the wrong kind of person knocking at your door or hauling you into court, then in many of these cases it seems like you're assumed guilty until proven innocent, even if that's not exactly how it's "supposed to work."

So, yes, closing down your wireless network is a smart thing. At a minimum, add a password. That's not completely secure - any password can eventually be cracked - but at least it's better than a completely open, public wireless network.

Ideally, though, you "pre-approve" every device that's going to use the network, and that's the purpose of the MAC Address. The trick is figuring out what it is on your two Macs...

You can crack open your "Terminal" application and use "ifconfig", but there's another way to accomplish the same result, depending on how you're connected to the network.

I'll start with the MacBook you have. Most likely, that connects via wifi / AirPort, so here's how to figure out the MAC addr. Start by going to System Preferences off the Apple menu, then choosing Network. Here's what you'll see:

You need to click on the "Advanced..." button, as highlighted.

Now you'll see this:

The MAC address is the "AirPort ID" shown on this window.

If you are running Ethernet - that is, hardwired Internet access - then instead of looking at the Airport ID, you need to click on the "Ethernet" tab along the top, then find the ID that's shown there:

Between those two you should be able to get the MAC addresses you need, add them to your router, and have a highly secure network again.