yo,
Same thing happened to me. You have to re-install it. What I did wrong was just selected the presets when asked waht folder to save and install to. The installer asks you this twice. The first time they ask for the main directory and then they ask for the vst folder location. For the VST folder location you must select the folder "VST plugins" which is in the Image-Line/FL Studio6/Plugins/VST
Select that and then your hard drive for the main destination and this should solve the problem. Anything else, I have no idea