Strange, that file was supposed to work... I've made one myself if you want to try: Planet Editor was broken
And I immediately have a huge number of questions, but could you tell me where to find a manual on changing worlds? I'm wondering if it's possible to make a planet with a retrograde elliptical orbit near the earth and an apogee somewhere far away? Crossing other orbits
There's no manual, but the text files are pretty explicit, you should find everything you want without too much pain.
For your planet you'll have to change this section:
Set direction to -1 to make it retrograde. For the orbit itself you can alter the semi major axis, eccentricity, and argument of periapsis.
The semi major axis (sma) is equal to (periapsis + apoapsis)/2. Eccentricity is 0 for a circular orbit, and positive for an elliptical orbit. It should range between 0 and 1.
If you want to set periapsis and apoapsis at a given value, you can use those formulas:
periapsis = sma × (1-eccentricity)
apoapsis = sma × (1+eccentricity)
Finally, argument of periapsis is an angle in degrees, it tells the game at which angle should be placed the periapsis.