This has been a long standing debate in the BIONICLE community since basically word go. I'll try and break it down a little.
Lewa has at least four common pronunciations. LEE-Wah, LEH-wah, LEW-ah, and LAY-wah.
LEE-wah is the most common, and mostly among North American speakers. It's my preferred way of saying it, along with Greg Farshtey's. It's also the way it is said in BIONICLE: Mask of Light and Generation 2.
LEH-wah is in a similar vein to saying LEH-go, and it's fairly straight forward.
LEW-ah is also fairly straight forward, since the first three letters are LEW with an ah tacked on. It's most common with folks outside North America.
LAY-wah is a manufacturing with no real sense to it. Greg Farshtey has said he wrote it that way in the guides because his Scandinavian co-workers would pronounce it in a way similar to "Leyva," and that was the closest he could get. He wrote it that way figuring they'd want that and he needed the guides approved. But recall, Greg himself has said he says it "LEE-wah."
So at the end of the day... It's up to you how to say it. No one really cares at the end of it all (even if I joke about it), and Greg has even said that while he considers the movie pronunciation the correct one, you can say it however you see fit.
Hope that clears things up!