1) There is no setting for English or Metric Units
2) There is no setting for spindle speed
3) The windows for the numbers are too small and the numbers can get cut off.
4) there ate two settings, wall thickness and the Xo and Yo that seem to be redundant.
1) You are correct, but just use appropriate numbers and add the G20 or G21 before you run the code.
2) Again, you will have to add that yourself.
3)You must be using metric values. I tried an English sample and the numbers fit ok.
4)Xo and Yo refer to the corner of the stock. The wall thickness lets you cut a slot of less than the whole stock wide.
I tried an example and it seemed to generate good code. Your comment
When I activate it, the machine seems to wander all over the place.
doesn't tell me enough to understand what you see happening.
These are user submitted, unsupported wizards. Kirin did a lot of them when the wizard capability was first added, but I have not seen his name on the list for a while. Maybe he has other interests now. Kirin did good work, and I'm sure the wizard works, but it certainly could use some additional features.
You can use the screen editor and fix these wizards any way you want to if you are willing to learn how to use the tools.
The Newfangled Set costs $50, but it is much more complete and is supported. There is not exactly a slot through operation but the surface or rectangular pocket would do the job.