Hi,
Good question. I wish that the GMAT would allow you to bold/underline or even take notes on the screen BUT the answer is no. You get the text on the screen and no extra annotation features. The best that you can do is to develop a good system for taking notes on the plastic dry erase pad that is provided. It's a good idea to get a lot of practice doing verbal questions on a computer and using one of these pads to take notes.
I hope that was helpful. Let me know if you have any more questions.
HG.