Since you have a deceleration, then the net force on the balloon must be acting upwards. So the upthrust, acting up, must be greater than the weight, acting down. The deceleration is the also in the same direction as the upthrust.
If you consider downwards as positive, then you could write your equation as
m'g - T = - m'd
which is just the same as
T - m'g = m'd