I am not 100% sure of this, but I believe Mikrotik is still working on USB routing implementations.
Mirkotik is a router OS, so essentially, you need to assign an interface name to your network card, and to your USB modem.
Set the interface name of the USB modem as the gateway for the network card, and remember to cache the gateway setting over to your DHCP settings as well, so that clients will use the network card as the default gateway.
A more specific explanation of your setup might help answers come more easily.