I think one of the legs is a bit short, and if I push it over, it wobbles. The other three legs are fine, and is pretty level.
I'm not totally sure what plumb means, but I'm assuming it means something relating to plumb-lines.. meaning vertical? I'll go take a picture.
Plumb means all 4 legs are perfectly vertical. You need to measure this when you place the stand on a spot that is perfectly level.
What do you mean - "it wobbles"? Is the stand wobbly? If yes, you have some fixing to do. If the frame is rigid, and the whole thing wobbles on the floor, you need to find out why.
First, measure the floor with a long level. If it is not, find a spot that is to test the stand on. In a pinch, put it on a piece of plywood & shim the plywood until you have a level surface.
Next, measure to see if the stand is plumb - ie vertical is all directions - check each leg in both axis.
Then, check that all your corners (all 24 or them!) are square. <Alternative - measure the 2 diagonals on all 6 faces. The diagonals on each face (& its mate opposite) should be within 1/8". (It should ideally be identical.) >
If any of the measurements are off, you need to correct the stand. I assume that you glued your joints. That will make correction difficult. If the stand is off by just a little - say 1/4" or less - you can belt sand it until the bottom is perfectly level (& the legs are plumb) so it won't wobble. Then belt sand the top so that is level with your floor. Anything more & you need major surgery.
Let us know how far off you are & we can give you more directions. In general, it is best to have your frame perfect & shim the floor where it sits if the floor is not level. It is bad practice to shim up an off plumb/ out of square frame, esp. when you have a lot of weight on it. The shimming puts your legs out of plumb & put stress on the joints.