I just received the MMD4E as a gift. Why doesn't it read anything when inserted into wood or even a wall.

The lowest level of moisture this unit can detect is 5%. Theres a chance the material measured is underneath this threshold in which case the unit wont respond in any way as it doesnt recognize that a reading is being done. Plug the pins into the two holes on the back of the pin cover to test the sensors calibration, the reading in wood mode should be between 17.3-19.3%. If outside of this range, contact techsupport@generaltools.com for a warranty claim.