For maps that you export from Zbrush you must have Uvs. When applying maps in Maya or any application without Uvs you have a choice of projection types like Planar, Cylidrical, these dont use maps, but rather wrap maps around your model a certain way.
So to export a texture map, you must of course paint your model, at this point your painting directly on polygons. Next you must copy this data to the Uvs which you must have on your model first. You Can apply Uvs in Maya, then bring that base model back into zbrush, or apply Uvs within Zbrush from the Uv map text menu to the right of the UI. I found PUV tiles to work well, your model must be at the lowest sub d level when applying these Uvs.
Once you have the Uvs, then you must copy the painting or Zbrush texture paint work into a map which uses these new Uvs made.
Before doing this you must sellect a size map you require, the choise is from 512, to 4096 pixels. Making shure you can see the painting on your model which means Colourize buttion is pressed in the Polypaint text menu, you then open the Text menu named Texture Map, amke shure the RGB is at 100%, and then press From Polypaint button once.
The texture map gets made, however if you look to the texture menu to the Left of the UI, you will not see the texture map there, this is because the texture map you have made is set for that Sub tool, where as the texture menu to the left is global places to load and export textures.
You must transfer your new made texture map to the global texture menu by click on the Clone textr button from the Texture menu to the right of the UI.
When you have done this you may now export your texture from the texture menu at the top of the UI first pressing the flip V button, then export.
Yes I know Zbrush does make you go around the whole UI to export a map, but thats how it works. It would be much easyer if you could export a map from the right texture menu. Im not shure if this covers what you needed to know, however should you want a video showing this, give me shout.