Text Align Buttons for Rich Text Fields
Per their configuration docs, which also link to Craft's docs on the subject, it looks like you'd just need to add "alignment" to the plugins portion of one of your craft/config/redactor/ JSON config files.

i.e. 

{
    "plugins": ["alignment"]
}


Then whatever config you end up modifying, you'll want to make sure that config is selected under that field's settings in Craft (Settings->Fields->Your Rich Text Field).