Use the select control located at the top of the page to assign a region number to the area you plan on hiding on the map. Next, make sure the check box labeled "Place Fog" is checked. Now click on the map and you should see a small black rectangular fog appear. Hold down either the "Ctrl" key or the "Shift" key, then click and drag on the fog to resize it. You can also click and drag the fog to move it any place on the map. Repeat the process until you have covered everything you want to within the region. Try to be efficient by placing as few fogs associated with each region as possible.
To hide other areas on the map, select a different region with the select control and repeat the process described above.
Hold the mouse cursor over the blackend area of the map you want to reveal. You should see a tool-tip display showing the region number. Select this region with the select control, then click on the "Uncover Region" button. If you wish to re-hide this region click on the "Hide Region" button.
Unfortunately at this time to display another map you will need to open the Map View.html file in any text editor, locate the <img> tag for the displayed map, and manually change the src attribute to point to the map you wish to be displayed. You can make multiple copies of the Map View.html using different names (i.e. My_Map1.html) and have each of these copies load up a different map image. Make sure any renamed copies you make of Map View.html are in the same directory as the original file. For convinience I like to store all my map images within the "Maps" folder located in the same directory as the Map View.html file.
There is quite a bit more functionality I would like to see added. Including...
Ideally I had envisioned this application to be used at the game table. A single large monitor, projector, or line out to a TV which all players and the GM could see. The GMs job is to load up the maps (with the regions pre-hidden) and uncover the hidden regions for the players. A wireless mouse would be handed around the table on each players turn to move their token on the map and declare their actions. But this is just me dreaming...