Re: So I bought Action Movie essentials...

It's pretty hard to get a random effect like fog to loop worth a damn, but here's my idea on how to get a loop out of it.

set up the fog layer(s) and render it out, or put it as it's own comp, whichever you think is better for your purposes. You won't need to tweak it.

add the fog to your original comp. enable time remapping for the layer.

set the layer to slow down to a stop about mid way through, then reverse the time back to the beginning. This can be done gradually enough as not to be noticeable.

or

put two copies of the fog layer, time reverse one, then fade between them at about halfway, or whenever it would look best to do so.

Unless your fog is blowing in one direction, this should work.

if it is blowing in one direction, then you might have to experiment with the evolution and see if there is a loop point that you can time to your animation.