Hi K.!
I've done this before, and boy does it ever STINK!!!
The first thing I did was to open up the house and turn on the fans--even strategically placing them so that they would pull the bad air out of the kitchen and fan it outside. (We left the windows open and the fans on for about a day too) I tried all sorts of things, from lemon juice,to baking soda paste and letting it sit overnight, to vinegar, to cleaning aids...to leaving baking soda sprinkled in it...to leaving an odor-eater in it too...
It just seemed like NOTHING worked...But, I kept at it...
If one thing wouldn't work, then I would try another... I honestly think that in addition to whatever cleaning methods you try, TIME is one of the factors that worked... (Kind of like "time heals all wounds", time eventually helped the smell to dissipate...I even check online to see what tips I could find and tried all sorts of ideas... Every time I went by the microwave, I would wiped it down with something.
God luck!!! I bet from now on, you'll keep a close eye on the popcorn. :-) I now prefer to use the air-popper. It's even healthier for you. :-)