You haven't followed all the instructions on http://isboxer.com/wiki/WoW:Interact_with_Target

Specifically:

  1. In the top pane, select Variable Keystrokes
  2. In the bottom left pane, click on Interact with Target under Variable Keystrokes
  3. In the bottom right pane, at the top, is a Default Key Combination option. Click the < to pick a Key Combination. Note: this must be the same as the key configured in the game for Interact with Target, which is G in our example.