http://www.matrixgames.com/forums/tm.asp?m=3817968&mpage=2&#3853978
by Jack Bechtold

1) Convert your map to a .bmp file
2) Open ODD. Click Map Tool button.
3) On menu bar at top click Import.
4) Browse to the folder your .bmp map file is located. Click on the file to high light it. Then click on the Open button. Your .bmp file will appear in the ODD Map Tool window.
5) At the top of the Map Tool window you will see a tool bar. On the tool bar is a hexagon in a button [tenth button from the left]. Click on this to scale your map.
6) When you click on that button mentioned in 5 a bar will appear at the bottom of the window.
7) Click the Set Scale button.
8) This next step is one of the most important. Locate two points on your map whose distance you are CERTAIN of. Usually the map scale bars that are on almost every map. Since I use 1:250,000 scale U.S. Army Map Service maps with a Transverse Mercator Projection and 10k meter Universal Transverse Mercator Grid I am able to use the 10km grid lines. You can find them at the U of Texas http://www.lib.utexas.edu/maps/topo/ . Military maps are the best way to go, Soviet/Russian military (my preferred) or U.S. military. So, back on subject. Left click the first point and don't let up on the mouse button. Scroll to the second point and release the mouse button. This will tell ODD how many pixels this distance equals and this pixel number appears in the first box after the Set Scale button. Now you are ready to fill in the remaining scaling boxes.
9) pixels = __ Kilometers you enter how many kilometers the pixels in the first box are equal to.
10) Starting X and starting Y leave empty. They will self fill.
11) The last box is where you enter the map scale. If you are making a 5 kilometer per hex map enter 5.0 here. 
12) Now click on the draw grid box. Tada. You map has a grid.
13) Click on the Export button on the top menu bar and save your map at a location you desire with a name you give it. IMPORTANT. Be sure to save it as NAME.BMP!!! DO NOT FORGET TO TYPE IN THE .BMP EXTENSION!!!
14) Some people complain the lines are too faint. So, copy the Starting X and Y numbers and the pixels = number. Leave the program. Start it back up. Load your map with the grid that you have already made. Proceed as above but since you already know the scale you don't have to set it. Just fill in the number, the pixels = and the Starting X and the Starting Y. Now reduce or increase the Starting X and Starting Y number by one number. Enter the scale and click the draw grid button. Since the grid is offset one pixel from original the lines appear darker. Save as before.