For installation of KeyTweak perform the following steps

(1) Go to the After Effects Preferences (Edit->Preferences->General in
the After Effects Menu) and make sure that "Allow Scripts to write
Files and Access Network" is enabled.

(2) copy the file KeyTweak.jsx into the folder
"Support Files/Scripts/Script UI Panels" of your After Effects
installation. 

(3)Restart After Effects and you will find KeyTweak in the window menu.

Here is a video tutorial showing the installation:
http://vimeo.com/15978374
