The requirements are that you need a host program capable to play them. The requirements are OS platform dependent. So you have to have or purchase a program that will load/play VST 2.4, VST 3 or AU sounds.
If you don’t want to purchase one then I suggest that you search the web for “Free VST Host”. You will find one.
http://www.steinberg.net/en/products/vst/dark_planet/specs_downloads.html
Regards 