I just created a new Unity project, and imported 2D Toolkit version 1.8b1. I created a new GameObject and selected it, then tried to add a tk2dTextMesh component to it. This caused an IndexOutOfRangeException error in the Unity console.
It turns out that I had not yet created a font.
I was able to fix this by importing a .fnt and .png file, creating a Font from the Project->Create->tk2d->Font.
It would be helpful if we could get a "Whoa there; you need to create a font first!" popup (or even a Debug.Log() message) to explain that the developer (us, not you) forgot to do that step.