Make sure you've started up Minecraft with Dynmap installed (Plugin or Mod). This should create a Dynmap folder in your Minecraft directory, everything will be done working in this folder!
It's recommended on some forums somewhere that you don't go any higher than x32 texture packs, I've completely ignored this and have it work fine with x64. I believe it's just down to hardware when rendering the map which is why this was previously suggested.
Open your Configuration.txt
and set the following option to hires.
line 16:
- deftemplatesuffix: hires