What is the best way to secure tank treds to a plaster base ,also is there a way to reproduce the effect of sand being thrown up behind atank while it travels threw the desert.

where are you in tank construction? if you still have ther hull split you can srew and bolt it down to the base. if not try epoxy, it will dry very hard, but test the epoxy on both base and plastic to make sure it doesn't affect either material...
as for the dust.......try pulled cotton it might work. other wise i've only seen it done as a photo effect with painted glass infront of the kit (shepard paines favorite effect). happy modeling