you should just be able to go into manual mode, keep dialing the shutter speed slower until you see "bulb"
this will keep the shutter open as long as the release is pressed. You can use a wired remote with a "lock" button so you don't have to hold it, and there is a more expensive version of the remote with a timer included.
I bought a cheap wireless remote for my D200 from gadget infinity and it works on my D300s as well. It is this one for $29.95. Works very well for me and I take long exposures often.
You have a D300s and you don't know how to use shutter speed..?
Was gonna ask the same Q...
D300 Manual >> Pages 110, 111 and 369 <<
Get an electronic copy from the Nikon Support site if you can't be bothered hassled about wading through the 400+ page manual that came with your camera, and use an electronic search through ADOBE READER, search for BULB, TIMER or SHUTTER RELEASE
I use DIYPhotoBits.com Camera Control. It's free software that you can use to control your camera with your computer, using your camera's USB connection. I'm not sure if you can do the bulb lock thing with it, but it's worth a shot before you go out and buy something really expensive. Also, I know the program script works with my D40, but I'm not sure if it works with the D300s. The sucky part is you have to keep the camera connected to your computer. If the program script works well and the only issue is cable length, head over to MonoPrice.com for a cheap USB extension cable.
