How can I put the Dashboard icon on my dock in Mountain Lion?
Any app that is currently running can be pinned to the dock by right clicking it, and choosing keep in dock, but occasionally you find a program like Dashboard or Mission Control or Launchpad etc that overlays the dock area when running thus not letting you add it.  The simplest solution is to find the application within your /applications folder, and simply drag it to the dock, where it should add a suitable shortcut that stays put.

However, I would generally advise for these sort of program that remembering the keyboard shortcut or trackpad gesture is often much faster and results in a less cluttered dock.