How to use a remote shutter on a Nikon D3100?

I just bought a remote shutter for my camera. I tried it inside and it worked fine, but when I took it outside to shoot star trails, I put it on Bulb, but it said it was too dark and wouldn't let me take a picture. I must be doing something wrong, but I don't know what.

What is wrong has nothing to do with the remote shutter release.

What you need to do is put the camera in the manual focus mode and pre-focus the lens on the night sky. Your cameras auto-focus system is NOT designed to work in the dark

With your camera on a tripod, focused on infinity and in the bulb mode, open the shutter with you remote and hold the shutter open for a few minutes. If the stars are too dim, open the lens aperture a stop or two. After making a few tests, you can then lock the shutter open as long as you need to capture the star trails

Note: Ambient light may cause lens flare or over exposed patches in your composition. Find a very dark location to photograph the sky

Set your lens in manual focus and put your lens focus to infinity, I know nikon does not mark their lens for infinity focus, So here is what you do, when you are looking at the back of the camera (LCD screen) and the lens is in manual focus, turn the focus on the lens clockwise all the way until it stops, then turn the lens back counter clockwise about 1/8th of a inch.