Here are some hints that I've found useful. They apply mostly to electrics, since that's all I've ever used the editor for.

1. To change the type of motor (like from gas to electric) click on the airplane's name in the lower-left panel, then on the right panel look for "Power Plant Type." The choices there are "Internal Combustion Engine," "Electric Motor," "Turbine Engine," "TurboProp Engine," and "Unpowered."

2. You cannot add/remove landing gear or make retractable gear non-retractable or vice versa.

3. Make sure you check the weight of the aircraft as you work. Some motors cannot provide enough power for a heavier plane. You need a power/weight ratio of a least 40 watts/pound for medium-sized electrics.

4. Gear Ratio is also very important. If your plane can't get off the ground, this parameter may be the reason why.