how can Irenicus cast dimension door and i can?

Because they game developers omitted the Dimension Door scroll in the game. It *is* in BG1 however, and you can get it with the CLUAConsole code (see the main page of the Ironworks BG2 section, click the Cheats link)

what does dimension door do exactly? where does it take you? what level spell is it?

As Melusine implies, in BGII, Dimension Door is just a plot device to get NPC's on and off stage. It was, IMHO, a crummy spell in BGI, and I never used it, playing as a mage. I admit, playing BGII as a mage, I wondered the same thing, when,spoiler
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after the graduation in the Planar Sphere, my apprenti DD'ed out.I just shook my head and thought,"I'm supposed to be teaching them, and they know a spell I don't." LOL