OK...you're obviously not a geotechnical engineer or you would know this, so I'll assume your handle is an attempt to categorize your problem, not a statement of your expertise.

If you know the moisture content and you know the optimum moisture content, the difference between the two is the amount of moisture you need.  Assuming that you'll compact to the max dry density, then the difference multiplied by the max dry density is the weight of water you will need.  Divide that value by 8.34 lbs/gal(if your density is in lbs/ft^3) or 1kg/l (if your density is in kg/m^3) (someone probably needs to check my conversion!)

As presented in your other post:

If you have one 2,000 pounds of soil with a moisture content of 12 percent and you want it to be 18 percent you do the following:  2,000 x (0.18-0.12) = 100 pounds

That's how much water to add.

f-d   

¡papá gordo ain't no madre flaca!

If you're trying to make compaction, bear in mind that while this calc will give you a rough idea, you're probably losing moisture to evaporation at the same time.