Which Alk did you buy? A dry cup is just that, a cup, however, when they say dry, they mean use a cup you'd normally use for dry ingredients (as opposed to, say, a Pyrex measuring cup). I just mixed mine today with the following instructions:

Alkalinity



*Note* Mixing the alkalinity portion directly in the jug is more difficult to dissolve and may result in clumps forming at the bottom. If clumps occur warm the jug in a sink of hot water and shake vigorously.

Fill one of the gallon jugs about 4/5th with RO/DI or filtered water and pour into the empty bucket or bowl
Measure out two level cups of Soda Ash.
While stirring the water slowly pour the Soda Ash into the bucket or bowl.
When dissolved use the funnel to pour the solution into one of the empty gallon jugs
Fill the remainder of the jug with water, cap and label "Alkalinity." Let the solution sit for several hours before use