You find the mean absolute deviation by first finding the mean, and then subtracting all the values in the data set from that, creating a new data set, and then you find the mean of this new data set.
Example:
3, 2, 5, 9, 4
Mean(add them all, divide by how many there are): (3 + 2 + 5 + 9 + 4)/5 = 4.6