Do you mean 1/30 (one thirtieth) of a second, or 30 seconds?
The longest shutter speed you can set in M mode is 30 seconds. If you can't set it below 1/30th, there is something wrong that I'm afraid I don't know how to fix. (IIRC there is a custom function related to flash exposure and aperture priority mode that limits the shutter speed - maybe that's turned on, and can affect M mode too?)