stockjaguar

There is no specific way to do it with the BAII plus calculator, you simply need to know the formula and go from there.
MAD = SUM [(X - Mean)]/N
The difference with MAD is that if for example your expected return/mean was 5% and you had a return of 4% thus equaling (4-5 = -1), you would then reverse your given answer from a negative to positive, in this case equaling 1. There are no negatives when using MAD.
Your ER = 6.8, lets use 7 instead.
= (4-7) + (1-7) + (8-7) + (10-7) + (3-7) + (15-7) / 6
= -3 + -6 + 1 + 3 + -4 + 8 / 6
= 3 + 6 + 1 + 3 + 4 + 8 / 6 (This is where you make any negative into a positive)
= 25/6
= 4.16 (may be a bit more as I rounded the mean to 7)