After installing the Discussion Forums add-on I went about creating my forum categories and quickly realized that they were not being displayed in the order that I wanted. I first thought that it might be set through the page order within the site tree but this is not the case. My next guess was that since the default categories were at the top they were being sorted based upon the creation date of the category pages and that turns out to be correct.
The way that it works is that the "older" the page the higher up in the list it will be. You can then use this to manually set the order in which pages appear by adjusting the creation date and/or time on your category pages.
I think that the easiest way to do this is to first arrange your category pages within the site tree so that they are in the order you want them displayed. This is just to have a visual reference to go by and make editing the pages in order easier.
Side tip, I would suggest setting the date the same on all of these category pages. This way you can focus solely on the time and use that to setup your sort order.
So now start with the top category page, go into its properties page and set the time to 1:00 AM. Move to the next category down and set the time to 2:00 AM. The next would be 3:00 AM and so on.
Again, if you have the date the same on all the pages it will then simply be keyed off the time.
Setting them in 1 hour increments leaves you plenty of space in between so that you can easily "insert" new categories in the future if needed. So you may set your new category to a time such as 2:30 AM in order to have it displayed 3rd in the list without having to adjust all the other boards below it. The one hour increments starting with 1:00 also makes sense in that 1:00 will be the first category, 2:00 will be the second, etc. I think it just makes sense that way but you can use this general methodology in whatever way you want depending upon the number of categories your forum has.