The other thing to consider, is jondecker, has done a lot of work with web admin, so upgrading the web admin may help.
http://forum.linuxmce.org/index.php?topic=5510.msg32690#msg32690 The only other thing that I can think of is that the device may be in the unassigned section. There is a drop down menu at the top of the AV devices page, which you can change the entertainment area. Try that, then the other thing I would try would be look in the device tree and see if the device was indeed added. If you can see it from the there you can change the settings from that page as well. You should be presented with a drop down list of available com ports, and you can pick the appropriate com port. Is there any way you could copy a screen shot in pastebin for us to look at?
the other thing to try is test from the command line: where $3 it the com port (/dev/ttyS0 or what ever you com port is)
/usr/pluto/bin/TestSerialPort -p $3 -P N81 -b 9600 -t "ka 01 01\r"
you should see the following if connected properly:
ka 01 01\r\na 00 OK01x