check out this site:

http://www.alpharubicon.com/bovstuff/tirepluguzi.htm

it provides a good discussion on how to use a tire plug kit. the plug kit pictured is identical to the one i have (and have used successfully on my wife's ford escape). and like the author says, despite the recommendation to have the tire inspected by a technician, the tire plug can be considered permanent. as long as the puncture is not in the sidewall they are safe to use and should outlast the tire - imho.