How can I make sugarless chocolate brownies more moist
That 1/4th cup margarine and the 2 eggs are not enough to give those brownies the moisture they need.  

I suspect you want something low in sugar and fat.  Evelyn Tribole, MS, RD created two books that talk about substitutions for fat and sugar in recipes.  One technique to replace fat (like butter) in a recipe is to use baby food prunes or applesauce.

Does it work?  I made her brownies that substituted baby food prunes for traditional butter.  The brownies were super moist and I couldn't taste anything "prunes".  This recipe is in her first Healthy Homestyle Cooking cookbook.

For right now, substitute cup for cup baby food prunes or applesauce for the butter/margarine.  I suggest half margarine and half the chosen substitute.  That way you get the flavor of margarine but without the fat.

Or, if you don't mind the fat just add more margarine.  The average brownie recipe usually calls for nearly a full stick (or more) of butter or margarine.  It's the margarine that adds moisture to brownie crumb.  Or oil, or any kind of fat you prefer.