Registering works fine for me. I just tried out the findme example included into the installation.
So it would be interesting how the register message looks like and to which port it is send.
Also, you can enable SIP message logging via the WLS Admin Console at the SIP tree so that the SIP messages are directly written to the serverName.log file. This should show whether the message is received by OCCAS 5.0.
Otherwise some network traces might give some more information.
Also, you can ask the same question on the support community forum for that product as well because there might be more people reading it: