I take it you mean adding buses to the in-route roads and not as driveable items on the track? To do so, you need a compatible bus (search for TomYoung on the DLS and make sure you select the buses marked traffic). To add them to your route, you need to find the region file in CMP or explorer if you're confident enough, and edit the config file. As I recall, all you need to do is select an existing vehicle and change it to the bus you need. In explorer, delete the asset from the config file and replace it with the details of the bus.
I'm sure someone else will provide a clearer explanation though.