You don't configure multicast address anywhere.
Some box sends traffic out with multicast destination, this will automatically get L2 multicast address on the link.
This L2 multicast address is normally broadcasted in L2, unless there are some specific features enabled, like IGMP snooping to stop the flooding.
If IGMP snooping is enabled, then the L2 switch will learn which ports want to receive the multicast (they do IGMP join to the multicast group) and will not flood traffic to other than interested ports.
If L3 is involved, then you'll learn via PIM which ports are interested in receiving the multicast.
I can recommend this book for multicast basics, it's not JNPR specific.
If you don't know which addresses to use, if you have 2byte ASN, you should use
233.0.0.0/8 GLOP block (with your ASN in it) if you don't have 2byte ASN your best bet is 239.0.0.0/8 which can be thought has RFC1918 (e.g. 10.0.0.0/8) for multicast.
mtftp would be what you're looking for. HOWEVER, you are very quickly going to learn wireless (esp. old 802.11b hardware) and multicast do not play well together. Outside Uverse circles, it's hard to tell what wireless APs can deal with multicast -- Uverse has wireless receivers fed from an isolated AP, but the limit of 2 STBs suggests they don't work so well either.
Best Answer
The 802.11 standard calls for broadcasts and multicasts to be sent at the lowest possible rate. That is because there may be receivers too far from the WAP to receive at a higher rate, and multicasts and broadcasts must reach all the receivers.
The original way to overcome this is to convert multicast to unicast at the WAP.
A Scheme Improving Performance of IEEE 802.11 Multicast Protocol:
Although sending multiple unicast frames in place of a single multicast or broadcast frame may be even slower, depending on how many receivers and the unicast rate.
There is now the 802.11aa standard, but it will take some time before both WAPs and hosts are compliant with the standard.
A First Implementation and Evaluation of the IEEE 802.11aa Group Addressed Transmission Service: