If you're using the "open" option, it will be in a temp directory on your computer. A better way to do it is to use the "save" option, which will download the zip file to a location of your choosing. Then you can unzip it and store it on your desktop or even right in the Plant Tycoon folder that contains your savegames (so that it's easy to find).

This post tells has step-by-step instructions for putting it on your desktop.