The easy way is to open Manager and check the status bar (bottom right), the TFTP log, and the BOOTP entries. These should tell you of any BOOTP requests from an IP Office on the LAN, and will list its IP address. Luckily, the units use BOOTP and broadcasting, so it should not matter what your subnet is on your PC.
Have you connected a serial cable to the unit and tried to manage it that way? You can always launch a console and default the unit using some simple X commands.
If you're lucky and SNMP is enabled, you could use a network sniffer to find the IP address, some sniffers even use broadcasting so you could try there.
Or, try the default - 192.168.42.1 / 255.255.255.0
Hope this helps