Installing Vigoro couldn't be easier! Simply extract the ZIP file wherever suits you best, and then copy the Vigoro.jsxbin and the Vigoro folders to your Script UI panels. The locations of these should be as follows:
PC: C:\Program Files\Adobe\Adobe After Effects CC\Support Files\Scripts\ScriptUI Panels\
Mac: \Applications\Adobe After Effects CC\Scripts\ScriptUI Panels\
Once you've copied the files, just open After Effects and open the Vigoro.jsxbin window (it should be at the bottom of your Window Panel at the top). Now you're good to go!
Note for CS6 & CC!
Vigoro uses 'Pseudo Effects' to create the nice control panels included in Vigoro. Inside the ZIP file you've downloaded is a file called [PACK] Vigoro UI Panels.jsx - installing this will allow these panels to run without any errors. If you don't want to install them everything should work just fine, but you'll get some errors about 'Missing Effects' when you create a new Vigoro Composition.
To install this pack, run AE and then go to File>Scripts>Run Script File... - Find [PACK] Vigoro UI Panels.jsx wherever you've extracted it, and run it. After it's installed you'll be prompted to restart AE. Upon reopening, everything should be working!
If you're running CC2014 or later, you shouldn't need to run the [PACK] installer!