It's way at the end. :) But she's fun to watch.

He opens rocking otu to Dr. Horrible. :)

She starts off by telling everyone to watch Dr. Horrible, then shows us her guitar. :)

DeathSun is working far from home and wishing he could watch Dr. Horrible. The mention is about 30 seconds in.

Later he makes it to the hotel and gives a little review of Dr. Horrible, after some here's my hotel room tour talk. Then micro review of Dr. Horrible is about 2 minutes and 18 seconds into this bit. Way near the end, but it's there.