It's nearly impossible to miss, being the only large-ish object in the game that spins until you pick it up. But as for where, it's in the room with the tablet that talks about the password. Unless you skipped acts, in which case it will be put directly in your natural path.
Except on Submissive and Breedable mode, where it doesn't exist. After all, "submissive girls don't need a weapon" :3