The windows steam Borderlands 2 can only cross platform play with the steam Boarderlands 2 for mac (and visa versa). As you have mentioned Game Ranger it sounds like you are not using the steam version of Borderlands 2 for mac.

> Mac App players are able to play with other Mac App players ONLY.  Mac App players will NOT be able to play with Mac or PC players using Steam.

[source]

[source]:http://support.aspyr.com/entries/22353786-Borderlands-2-FAQ-