[maxerickson]

Tilemill ingests geographic/spatial data, it doesn't have much provision for interacting with it.

You might find that someone has already drawn the bus routes you are interested in:

http://wiki.openstreetmap.org/wiki/Buses

(Then you just have the task of pulling the data out of OSM and translating it into a format that you can work with)

[Demiurge]

Also, I just realized, if the bus routes are not in the OSM, he could use the new iD editor and add the bus routes to OSM under his account, and then export :)