The last time I remember it showing up was when Ben pulled that plug or whatever and it came up through the drain.

I don't think it has been fully explained... I have to admit I had huge expectations for it and it does not look like it is going in the direction I envisioned. The first few times we saw the smoke monster it sounded very mechanical. Remember when it was pulling Locke down the hole? I thought it sounded like a mechanical crank, roller coaster-ish even. I really thought that it was going to be logically explained. I was thinking something like the Wizard of Oz, a man behind the curtain.

Other than that we know that Ben has the power to summon it. When the military guys had Alex at gun point, he ran down in his basement put his hand up in some shelf and the smoke monster came and dominated the militia.