If you are viewing the LCD before taking the photo and it is appearing black, you are looking at what the image would look like if you took the photo with the current setting. It does not account for the light contributed by the flash. To disable this feature go to the Setup menu page 1 and choose Screen Setup then set Preview exp in manual mode to OFF.
If you are previewing the LCD after taking the photo, the manual setting you picked underexposes the image severely. Increase the ISO, use slower shutter speed, move the flash closer to the subject or increase the output of the flash.