To enable half-star ratings, close iTunes, and open Terminal (/Applications/Utilities). Once you have Terminal opened, type the following command and press enter:
When you reopen iTunes and rate a song, you will be able to give half-stars. That simple. If you wish to make things normal again, open Terminal and type the same command, replacing "TRUE" with "FALSE."
defaults write com.apple.iTunes allow-half-stars -bool TRUE