As far as I know, we were never given the Kent's mailing address. (Unless they showed the house number on the house at some point. I don't recall seeing that, but I could have easily missed it.)
As for how it works in general, I promise you there are streets. Even though farms are far apart, you have to take a street to get to the driveway. People don't just drive over fields. So they live on a street and have a regular address. The street may or may not have a name. It could be 316 Johnson Pond Rd. or just 316 RR 12 (RR stands for Rural Route). It's also possible that they live far enough out that there is no mail delivery. In that case, they still have a street address (in my make believe world here - 316 Johnson Pond Rd.) but they also have a mailing address - a post office box. So their mailing address would be something like P.O. Box 125 Smallville, KS.
Sorry, I can't be of any help on the kryptonite question, but for the record, it's always okay to create your own - that's the fun of writing your own story.
Annie
