In the last tutorial, Sean mentioned that there is something with the size of the image. You need to adjust the horizontal and vertical down such that when you hold down the shift key, you see a yellow box where you would click and hold the mouse. If it is somewhere other than the mouse pointer, you need to make the image smaller. If you don't do this, the mouse isn't pointing where you think it is, and you can't grab things like the single strings you are trying to do. Larger objects might still be ok, because they are so big, but smaller ones will be a problem.
