You’ll need two files: keys database and AACS dynamic library.

Keys database

get the file UPDATED 20-04-2012!
Windows: put it in %APPDATA%/aacs/ . How to find where %appdata% dir is?
Mac OS: put it in ~/Library/Preferences/aacs/ (create it if it does not exist)
Linux: put it in ~/.config/aacs/

AACS dynamic library

VLC 32bits on Windows: put that file in your VLC directory
VLC 64bits on Windows: put that file in your VLC directory
Mac OS: put that file in ~/lib directory (create it, if it does not exist)
Linux 32bit: put that file in VLC folder (or in LD_LIBRARY_PATH, usually /usr/lib/)
Linux 64bit: put that file in VLC folder (or in LD_LIBRARY_PATH, usually /usr/lib64/)