you might haveto install your vsti into your fruityloops plugin folder imageline>plugins>vst's ..< in that folder, then once u done that open FL click on Channles & do a quick refresh & it should be in there
it doesnt have to be in your fl studio plugin folder. as long as you had to do an install just refresh your plugin. you only need to put it in your folder if it wasnt a file you had to install. like a .dll that you just download