"We step down beside the bed."
Translation:Lelépünk az ágy mellé.
That is correct. It probably should be accepted either way.
But from a practical standpoint, mellé makes more sense since you're only ending up next to the bed, maybe by stepping down from a ladder or a short stair. If you say mellett, you'd have to spend your whole moving experience beside the bed, and I have a hard time coming up with any situation where that could happen.