You have to use arp. Remember that MAC addresses are used at layer 2 and IP is layer 3. You will only be able to see MAC addresses that are in the same broadcast domain (on your subnet). The easiest way is if you have a list of MAC addresses, then ping the broadcast address(x.x.x.255 for a class c) of your subnet. Then if you do an arp -a, you will get a list of all the ip's along with the MAC address for you subnet.