How to set load balancing with 2 or more USB Modem?

I have one router hap series, one powered usb hub and 2 or more usb modem. I want to make load balancing connection with those usb modems, how to do that?

http://wiki.mikrotik.com/wiki/Load_Balancing

that link only covers static ip on wan interface, not dynamic ip with ppp interface.