Only to expound on what Bob O said - the arp table is the way to go but will only be populated on a PC if there has been recent device-PC comms. If you just plug in a laptop to the network, chances are you won't see it. If you have been talking to it, then it will show.
Some other options if the arp table doesn't have it:
1. Set up Wireshark and sniff the ethernet frames off that line. Helps to have a managed switch with port mirroring (you DO have a managed switch, don't you???) but even if you don't all is not lost. Power cycle the device and sniff with wireshark on the regular switch and the device will produce Gratuitous ARPs on bootup which will announce the MAC and IP address. These are broadcast so even a switch will send these to every port. Just power cycle the device and that will cause bootup. A hub instead of switch will easily show all network traffic passing through with Wireshark, so is actually easier.
2. Use canned software to map the network - Schneider has something along those lines and since you using their hardware the local sales guy may loan you a copy or something.
3. Write a script to ping every device in the network... looping through. Only suggest if (1) is not possible.
