I finally got the Adafruit Trinket to blink correctly as per the How to Guide.
I was getting errors on the upload, until I found an FAQ that explained that Windows takes more than 10secs to compile the sketch.
The solution was to click upload, then wait about 5 secs before clicking the reset button.
I don't know if this solution will work for a longer sketch.
So Trinket's a bit frustrating for me so far.