I probably do sound like an idiot when I ask about the reservations on this one but the CM seems to know what I'm talking about (and often pronounces it the same way).
I pronounce it: Le Sell'-yer
Since it's basically French (I believe) for cellar, I just assume the C makes an S sound and then pronounce the ending based on English pronunciations of ier. I would assume that it's correct pronunciation is actually: Le Sell-yay'. (The ' is the stressor, in case you were wondering.)
Not sure if that helps or hurts. I wouldn't be too concerned about sounding like an idiot though. Most likely they won't add that information to your reservation and, if they do, you might get a free dessert!
Le (as in "look") Cell-YAY (with the emphasis on the YAY!).
(Actually, this is kind of a running joke with our group. We jokinglycall it "Le Cellular." One of these times, I'm going to forget and pronounce it wrong!)