D90 AF assist light doesn't come one in M mode???

Did you switch the Focus-mode selector to MF?
It is the switch with number 8. Perhaps you are used to switching on the lens, but on the body is also a switch. Normaly used for the older AF lenses.

Does it shoot in manual focus? In which autofocus mode(s) does it not shoot? Are you trying to focus too close, or on a subject with insufficient contrast?

The D90 has three different autofocus modes. In the AF-S mode it will not shoot until it has focused. In the AF-C mode it will shoot when you press the shutter release button even if has not focused. In the AF-A mode the camera switches between the AF-S and AF-C modes so either can apply.Also, bear in mind that the camera doesn't really know what you're taking a picture of. It may focus on something other than your subject. If you're in the single-area AF mode then you control which of the eleven focus points the camera uses. In any of the dynamic-area AF modes the camera decides.For full details on how the various modes work, please consult your manual. You might also check out the Nikon D90 video tutorial.

With such an old lens, you'll have to use it in Manual exposure mode. Turn the mode dial on the left side of the camera to M. Control the shutter speed by turning the main command dial on the back, the aperture by turning the ring on the lens. You will get no assistance in metering from the camera.

Try cleaning the contacts on the body and the lens where they meet with a cotton swab lightly dampened with rubbing alcohol.

DEFINITELY return it while it iis under warranty.

keep the camera mode MANUAL

Custom Setting Menu A, autofocus, setting A3 "Built-in AF-assist illuminator".

174 in my D90 manual.
"Choose whether the built-in AF-assist illuminator lights to assist the focus operation when lighting is poor"

Which mode are you shooting in?
Some modes will disregard your af assist option.
Try a different mode see if it works.

If not MSG back.

Please rate.

There are four focus modes: S-AF (Single auto focus), C-AF (Continuous auto focus), MF (Manual Focus), and S-AF/MF (Simultaneous mode). (SINGLE Auto Focus): Focusing is performed once when the shutter button is pressed halfway. C-AF (CONTINUOUS Auto Focus): The camera repeats focusing while the shutter button is pressed halfway. (MANUAL FOCUS): The function allows you to manually focus on any subject while looking through the viewfinder. SIMULTANEOUS USE of S-AF mode and MF mode: You can fine-adjust focus manually by turning the focus ring after AF is performed in the S-AF mode.