Hey Vincent... sounds like you're already half way there... the thing about 'Fishfillets' is that they put their plugins into their own folder so if you've installed them into your VST Folder they will be in what's called a Sub-Folder... so go to Preferences>Effects and click on 'Add' this will open up a 'Browse for folder' option :
scroll down till you see the FishFillets folder and click on it... then click OK to all the boxes till you're back to the main program. Generally Mixcraft will have already put the VST plugins into the list but you may need to restart Mixcraft first... it's worth checking first tho.
Next click on the downward arrow where it says <Select An Effect> and a pop-up list will appear with all your VST plugins in it:
Scroll up till you see your plugins and click on the one you want. Once you've got it .... it will appear in the Effects section of Mixcraft... to bring the plugin up onto the screen click on the 'Edit' button. One other thing to check is that you've clicked on the 'Track Tab' (lower left hand side in Mixcraft) otherwise it will apply the Effect to the whole project!
Hope this helps.... you probably already knew most of it but it might help others having a simalar problem