How come my lens won't autofocus?

I bought the Nikon Nikkor AF 50mm 1.8D for my D3000
It's only manual focus but it says it's automatic and manual focus

Added (1). There's no switch for AF l MF. My 35mm has it but not this one.

There's a switch on the lens barrel. What's it set to?

The Nikon D3000 lacks an AF motor built into the camera body. The D3000 will only autofocus with AF-S lenses.AF lenses have the capability of autofocus, but only if the camera body has a motor.

Did you not read that ONLY AF-S lenses will auto-focus with Nikon entry level dSLR's?

In order to keep the price for D3000, D3100 and D5000 under $1, 000, there's NO auto-focus motor in those cameras. That means that only AF-S and AF-I lenses can be used in the auto-focus mode because they have auto-focus motors in the lenses. This is what makes AF-S lenses auto-focus so quickly.

Either return the AF 50 mm f/1.8 and buy an AF-S 50 mm f/1.4 or learn to manually focus using the built-in electronic rangefinder

All this and more is located in your user manual

Your lens is an AF-D. D means drive, requiring a drive motor in your camera body.

The D3000 has no drive motor in the body, so you can't use AF on it.

If you need to use AF on it, you must have a camera body with an internal drive motor, like the D90, D7000, D300(s), D700 or the D3(s/x).

By the way, good choice of lens though. It is one of the best lens you could buy with a limited budget.