NET-OS 7.4 and UDP multicast sending

I have a very big problem with my ethernet application developed on connectme 9210 platform with Net-OS 7.4.
I tried to send multicast packets to a multicast group via udp socket, but it didn’t work and so I used a ethernet sniffer to look the packet over the network. I found that the multicast packets were sent, but with not correct multicast MAC address (unicast mac address was encoded into ethernet packet). ?Anyone could help me I need to solve this problem quickly.
Thanks a lot and sorry for my english.