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In Tenacious D - The Pick of Destiny we learned that the devil got banished from Earth using the spell

From whence you came

you shall remain...

until you are

complete again.

But later we found out that the boss of the music club is in fact the devil in disguise.

So I am wondering, how did he get back to Earth when he was actually banished? He was banished until he is complete, but he was on Earth already before he got the Pick of Destiny.

The guy may not be the boss of the club, he's just billed as "open mic host", played by Paul F. Tompkins. The thing is, he was also a regular on the HBO Tenacious D series collected on the DVD Tenacious D - The Complete Masterworks. And Pick of Destiny was basically a prequel to the series, showing how Jack and Kyle first met and formed the band--so if there's supposed to be some continuity here, I think we have to assume that the open mic host was a real person, and the devil was just temporarily impersonating him.

So, my theory on this would be that even in the movie, most scenes with open mic host were the real guy, and only after they had broken the pick did the devil take his form. When Jack and Kyle first come back with the pick he's outside smoking, he tells them there's one spot left and they tell him "What we got's gonna turn your brain to shit! Dust off the stage, open mic host. Step aside." He responds "OK big shots, you're up whenever you're ready" in his usual deadpan style, and then steps inside. Then they fight over who gets to use the pick, break it, and then open mic host steps back out and says "you guys, the crowd's getting restless in there--hey, what's--hey, hey, hey, what's wrong?", acting all concerned about seeing them sobbing on the ground, and when they explain about the pick he gives them the inspirational speech about how satan is in their hearts. So from the start, when he comes back out he seems kind of out-of-character, since he's normally pretty indifferent towards them. This would fit the idea that they first saw the real open mic host smoking outside, then when the pick broke the devil was immediately able to take his form and pretend to be him.

Also, when they were fighting over the pick it was glowing green, then as soon as it snapped in two the glow immediately faded, which you could maybe take to imply the magic had left it, and the spell cast long ago by the wizard using the tooth (which was carved into the pick) had been broken or at least weakened. Even in open mic host form, the devil still apparently needed to get the pieces in his hands to transform into his full majestic Dave-Grohl-in-red-makeup form, but maybe you could imagine the breaking of the pick gave him some kind of permission to come back up and collect the pieces to become "complete again".

Or maybe his ability to influence the surface world was a matter of degree--like, even when the pick was intact he could do things like influence the minds of men (as implied by the final lyrics of the song "The Metal" which played in the credits--"Metal! It comes from Hell!"), so when the pick was broken his influence was greater even if it still wasn't at full strength, and that allowed him to come to Earth in a relatively powerless human form. Or maybe you could imagine the form of the open mic host was just a kind of psychic extension of himself, while his real body was still confined to hell until the psychic extension got the pieces back. It's magic, who can say exactly how it works?

But either way, I don't think he would have been able to appear on Earth in the form of a human while the pick was still intact (if he could, he could have just appeared in the dressing room of Jimmy Page or Pete Townshend or any of the other previous owners and grabbed it then, or manifested at the Rock & Roll History Museum when it was stored there).

This is not really covered in the movie,

but given the nature of the humor involved, once 'Tenac' on JB's backside and 'icous D' on KG's backside where placed next to each other, something unholy had become complete, and unleashed upon the world.

I know I can't unsee it.