To get a salt (and I suppose a butter flavor, if you are so inclined)
 you can put the popcorn kernels in a jar and add mixed salt water.
 Add just enough so that you can coat all the kernels.  You can
 leave the jar closed for a couple days, or if there is too much moisture,
 you can drain out the excess.  Then just put the popcorn kernels in your
 popper (microwave or hot air) and it will have a salt flavor when popped.