It is probable that you bought the wrong kit. Expressit comes in Windows and Mac kit versions, so you have to be careful on which kit you get.
All kits come with templates for software that can be used on a Mac, but the actual exPressit software for Mac only comes in the Mac kits.
You can get labelling software for Mac as a Download from the Pressit.com website, although we recommend people use our liteLabel and Disclabel software now for Mac users.