How to moisten a dry cake?
Soak it in booze!!
Serve with LOTS of whipped cream.
Ice cream cake!
Rum balls! (if all else is lost)
If it can be cut into layers, brush as much simple syrup - booze of choice mix as you desire, fill with a custardy filling such as chocolate pastry cream, wrap it in plastic and refrigerate overnight. Then pour a ganache glaze over the top. Enough gooey chocolatey goodness, and they'll never notice the cake.
some cakes, like genoises, always need to be moistened after baking. you can either brush on a simple sugar solution, or flavor it with your favorite liqueur (kahlua, grand marnier, chambord, etc.). brush the solution over the top with a pastry brush and let it soak in. then you can do whatever you were planning to do to decorate/serve your cake.