nikon d90 – Why is exposure meter reading not changing in Manual mode?

The manual exposure meter only displays when it’s helpful to do so. Mainly when you have the camera in Manual (M) mode.

In S, A, P modes, the camera is calculating the exposure so you don’t need it and it disappears.

It reappears in those modes only if you use the thumbwheels to alter the exposure past the range of valid apertures, shutter speeds or ISO. For example, in low light, if you have the camera in S mode and increase the shutter speed with the thumbwheel, the camera will compensate by using a larger aperture until it reaches the maximum aperture, then will adjust the ISO to the maximum. If you keep increasing the shutter speed, it will run out of options, reach max ISO and aperture, and that’s when it displays “Lo” and the exposure meter appears to show you that you are now underexposing the image.

Imre is probably right that you have the exposure set past the limits of that exposure meter. As suggested, put the camera in P, S or A mode, note what settings the camera has chosen, then put the camera in M mode and change to those settings. You should then be roughly in the center of that exposure meter.

What lens are you using? If it’s an older lens the camera might not be able to meter properly. Is it an AF-D or AF-S lens?