What can make my coconut cake moist......?

make some simple syrup -- equal parts water and sugar and boil till sugar dissolves -- then let it cool -- you can add rum or rum flavor or vanilla or whatever flavor you want -- or coconut flavor -- or none -- and brush or squirt it onto your layers as you build your cake -- 

also if you can trim off the edges that will help too

What K8memphis said.  But also, it would be helpful to know why/how it's dry.  Is it the recipe?  Is it overbaked?  Is your oven running hot?  Wrong size pan? Wrong kind of pan? Forgot to add an ingredient?

Now, if you just need to rescue a layer cake, if it was me, I'd torte it extra thin, use a simple syrup, and fill with a really lovely rich coconut buttercream, not too thick as that would overwhelm the thin layers but a nice generous amount.

yes what justonemorecake said -- and if you have time to freeze it after you apply the simple syrup -- that will help distribute the moisture throughout better -- i usually do this before final icing -- which is why i recommended slicing off the edges so they don't rob any more moisture from the center -- because typically they are the driest part --

and how's it going?