I'd just finished the lesson on blending in with the crowd and was told to go find Da Vinci. However, his workshop never showed up on my map. Does anyone know how to fix this?
Resetting the game (or I think AC2 has a reset to checkpoint feature that might work) usually reset the mission in progress if your mission has glitched out.
Been a while since I played AC2 but I also think Da Vinci's Workshop is not a location that is on the map, except when a mission needs you to get there (and it will just be your objective marker). I don't remember any mechanic where you had to get back to Da Vinci's workshop outside of a mission to do whatever. So that is probably why it is not shown as a 'neutral' marker on the map.
So continue on following the 'Next objective/mission' marker and it should be fine.