Hi, @katharine.e.kenyon!
In the last time I used Memsource, it was at the bottom (see the red arrows):
As you can see, there is no percentage, just the number of words, segments, and characters.
I don’t know if there is a button to display that, but if you really want to know the percentage, just divide the number of translated words by the amount of total words. In the example above, the translator finished only 8 words from 3,459, so 8/3,459 = 0.002 = 0.2% (approximately).
Happy translating! 