It should be an address in the multicast range, and the exact numbers designated will depend on your network and how you manage multicast. Avoid any addresses already in use by another multicast application or for things like routing protocols, topology discovery, etc.
You will configure 1 address to go in the RSM box on Symposium. That addresss will also go into Web Client as its "receive" address, plus a second address for Web Client's "send" address.
There is a section in the Web Client NTP that discusses multicast in some detail, recommend you read through it before you try to implement; should clear things up a bit.