files that end in .vst generally cannot be opened by clicking on them.
what you want to do is run the VST.exe program itself.
if you really want to be able to start up VST.exe by clicking on .vst files, you can google how to associate file extensions with the program you want to open them, but i would advise against it. while doing this will cause VST.exe to start up, VST.exe will ignore the .vst file you chose and instead do what it always does, default to the .vst file of the currently being played PT game.
so, to recap, find where you downloaded VST.exe to, and run VST.exe directly, not the .vst files that store the seeds. if you cannot find where you downloaded VST.exe to, download it again from the guide section of this forum.
my personal preference is to put VST.exe in the same directory as plant tycoon, and then create a shortcut to it on the desktop.
most professional programs come with an installer that would put VST.exe in the correct directory and create a desktop shortcut and even a start button entry and other fancy things. but, sigh, all i have to work with is the free version of visual studios 2005, which does not include those bells and whistles.