I've been digging around in the documentation.http://wiki.linuxmce.org/index.php/How_to_add_your_own_GSD_device
If your Ruby code contains an error, LinuxMCE may decide to disable the device. So, check that your device is not disabled: on Wizard-->Devices-->Generic Serial Devices click on "Advanced" for your device and make sure the Disabled box is NOT ticked. If it is, untick it, save, and do a quick reload of the router.
Turns out the device was disabled. I guess that might mean an error in the ruby code on the template. Unticking the box and reloading the router allows me to send commands to the device. I'll have to get it off my test bench, plug things in and see if it is completely functioning now.