From my experience, you can't be standing on it- just on the tiles next to it. Also make sure no enemies are aggro to you at all. That's worked in my experience
Don't stand on it, just near it. Then interact with it by using the tool bar. If others (like enemy) are interacting with you may cause nothing happen when you choose the bed. You can only use the bed once per floor unless you are in prison. If you are captured by enemy and in prison, the bed in prison could use more than once, but it need hurdreds of turn to cool down.
To answer the question on how to sleep in a bed or interact with other furniture. use the interact button which is 'B' You can use this interaction to put yourself into other furniture if you so desire as well.
