Discovering a device by MAC address

I have just plugged a GoFlexHome unit into my LAN and am trying to figure out if it has connected to anything...I haven't messed with it or installed Seagate's software to configure it but have noticed its blinking green light has changed to steady. This, according to the docs
Connection to network is normal

However I don't see it on pfSenses list of connected devices. I do have its MAC address so can I see if it has connected to anything?
Depends on your network topology. You probably use switches to connect several devices together and most of those keep the network traffic isolated between devices. So unless the device is actually broadcasting or making connection attempts to your diagnostics device it remains to be seen if you'll discover it.

Either way, one good way to check is by checking the ARP cache. So arp -a. See arp(8) for more possible uses.
On many of traditional IOS Cisco switches/routers you can do this:
show mac address-table aabb.ccdd.eeff
show ip arp

Other switch/router manufactures worthy of being in a network will have similar types of commands.

Assuming you are using DHCP server you can see what IP address if any was handed out. GoFlex devcies requires DHCP so if you don't have that up and running, now is the time to start reading/configuring.