I'm not entirely sure if the option is available, but you might be able to change the exposure compensation using the menu entry on the back LCD.
If it works, then it won't do much to change the actual exposure. That is still determined by shutter speed, aperture and ISO, which are all set to fixed values. It may change the "calibration" of the light meter though. E.g. if you set an exposure compensation of +0.7EV and you actually overexpose by 1 EV, then the light meter might only show +0.3EV, as that's the difference between the measured exposure and the set exposure compensation.
Exposure compensation in manual mode also will have an effect if you shoot with ISO auto on. the camera will select different ISO values then. One might argue that that's not true manual mode then though.
For all this i'm going by my experience with a D90, i didn't try it on a D3100 myself. I know for sure that i can set exposure compensation on my D90 in manual mode (for which i don't need the menu), and that it affects the light meter and ISO auto this way.
To whoever gave the thumb down: If you're a Nikon guy, try for yourself. Exposure compensation in manual mode does something to the calibration of the light meter, and it also affects the value chosen by ISO auto if that happens to be activated.
Sources which confirm this independently:
Another thumb down? Some people are being dogmatic, it seems.
I just checked again on both a D90 and a D40. Setting an exposure compensation will affect the value displayed in the light meter, as well as the value chosen by ISO auto. Like it or not, that's the way it is.You need not use it if you don't like to.