Reversed Ring Issues for Nikon D3100

I just bought a reversed ring mount for my DSLR, The Fotodiox 52mm Filter Thread Lens and for some reason when I attached it and turned my camera on, the screen reads "Lens Not Attached". When I clicked on the info button it read "Check Lens. Non-CPU lenses can be used in Manual mode only" So I switched it to Manual by moving the slider from "A" to "M" (maybe that's not necessarily switching it to manual… I don't know) So what's wrong with it and how can I fix it so that it will take pictures?

Added (1). @Jens: Thank you!
That helped, and about the manual mode, I of all people should know that cause I use it so much lol
Brain fart I guess.

Are you referring to the switch on the lens? That's the switch for autofocus and manual focus. You should set that one to manual focus too when reverse mounting a lens, but it's not what that instruction referred to.
The manual mode that you must use to use a reverse mounted or non-cpu lens is the manual mode on the camera. That's the M mode on the PSAM dial on top of the camera. Note that that has nothing to do with focusing, but with setting shutter speed and aperture.

Are you using the kit lens? In that case you'll have quite some difficulties setting the aperture when it's reverse mounted, as it doesn't have an aperture ring. You will have to block the aperture lever with a piece of cardboard then.