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just keep it in your bag, it randomly appears on floors 36-98, and holding A+B doesn't make it appear, if it isn't a stating pokemon on that floor, its not there.
A tip for finding Mew is to watch out for a text box saying, "Mew used Transform!" or, "Mew picked up the _____." Also, since Mew can look like any Pokemon, destroy anything that gets in your way. And just keep the Music Box in the toolbox.