I've been a personal trainer for 11 years.
There are very good supplements out there. Herbalife is not one of them.
You'd get better results eating the container. I've seen the reports of independent tests done on most of the major brands. Herbalife scored right at the bottom.
If you want to gain weight invest in one of the bodybuilder weight gain shakes. Just watch your fat gain.
In your case I'd say go for one of the cheaper ones. Ask someone at your local supplement store to help you.
Or even better I'll help you with a cheap and effective option. Make a shake yourself.
I use the shakes from this site:
http://www.criticalbench.com/proteinshakerecipes.htm
Actually all the recipes will help.
For all of the shakes I add one serving of oats. Just grind it in a coffee grinder or it makes it lumpy.
I also eat alot of oats. It's one of the best weight gain foods.