I finally managed to export the song I've been working on from Cubase into an MP3 file. I did it by first doing an Export Audio Mixdown to a WAV file, and having it imported back into my project as an audio track. Then I normalized the audio track.

Then I did an Export Audio Mixdown from the audio track into an MP3 file.

I ran into some puzzles, though:

1. I wanted there to be a short period of silence (~2 beats) before the MP3 started to play. I noticed there is a Start parameter in the Project Setup dialog, but I didn't really see how to use it for this purpose. So I ended up inserting the delay by selecting the Event containing the audio data from the creation of the WAV file, and moving its start point later by two beats. I'm wondering if this is the best/usual/easiest way of doing something like this. I suspect something more convenient is possible.
2. I normalized the audio track from the creation of the WAV file, but the MP3 file needed to be normalized despite that. I was able to do this using Audacity, but I was surprised that I needed to do it given that I had previously normalized the source track inside of Cubase.