How to take pictures in low light with a d90 nikon?

The manual says to hold down the shutter release button until you have enough exposure. I bought a minolta in 1985 and in auto it would release the shuttter automatically when it had enough light. I can't believe that there's'nt the same mode on the D90.

How little light is it that you're talking about? The D90 takes timed exposures according to the light meter/manual shutter speed setting of up to 30 seconds.
Beyond that, in bulb mode, you indeed have to take things into your own hands with a stop watch. Is that what you're talking about, exposures beyond 30 seconds?

You have to use your trusty light meter to get a proper exposure. It seems that you haven't learned how to correctly manipulate shutter speed, ISO, and aperture to conform to what you want to shoot. I suggest read a book or take a class/workshop

For tripod mounted exposures of up to 30 seconds you can avoid adding camera shake from touching the shutter release button by setting the timer to at least 5 second. That will allow vibrations to settle down.
Or you can use the optional ML-L3 Wireless Remote Control. If you shoot bulb exposures of up to 30 minutes you definitely should use the remote.
At BH Photo it costs $14.50.
http://www.bhphotovideo.com/..._4730.html

Have an extra battery with you. Long exposures can drain the charge a lot.