I installed them on my Mac, which is really simple to do:
- Download liberation-fonts-ttf-3.tar.gz
- Double click to unpack them into a folder
- Go into the folder and read the LICENSE.txt
- Click on the individual .ttf files to open the font collection app
- Check in preferences if you want to install the fonts for all users or only you
- Click on install
And you're done ... The liberation font(s) can now be used as any other font.