Well, I've been working on the keyer. It turns out that the Arduino Mega has the I2C bus on different pins than the Arduino Uno. Since my keyer shield doesn't actually have pins for that location on the Arduino, I had to come up with an adaptor mechanism. I bought some extra long pins from Jameco and put two of the wires into a two-pin housing, and now I have a display. The lack of sidetone output was traced to a broken wire going to the speaker. The paddle inputs didn't work, but that turned out to be a software error.