When you say "M3xx Media Server", are you referring to a TelePresence Server Multiparty Media 310/320?
Do you use a call control server such as CUCM or VCS? If so, you'll register the TP Server and SX10 to it, and call into conferences hosted on the TP Server. If you're not using a call control server, than you'll have to make calls into the TP Server by calling alias@IP_Address, where IP Address is the IP of your TP Server.
You can't register the SX10 to the TP Server, which is why it isn't an option, the TP Server is for hosting conferences only.
If you need more help, suggest you tell us more about your environment so we can get the bigger picture of what you're trying to do.