Are you just taking a weight gainer or are you training?
If you just take a weight gainer, and don't work out, what's the point? Your just wasting your money on a supplement your not utilizing.
With that being said, Muscle Milk is an expensive choice, so I do hope you are training. Take it pre-workout, and post-workout, and if you want you can have a scoop before bed.
If your not training, don't waste money on supplements!
Gaining weight is real simple, you consume more calories than you expend in a day. However, try to make those calories, quality calories, not junk.
If your not gaining weight, you simply are not eating enough calories. Each week up the amount of caloric intake by 100, until you start gaining, then level it off.
Peace