I’ve lived in Ethiopia (in Bonga, just 3 hours past Jimma) for most of 2011 and 2012 and can share some tips:
Both Selam Bus and Sky Bus travel the route between Addis and Jimma, but stick to no consistent schedule and frequently cancel buses the morning of departure (and expect you to just stick around for another day in Addis, as they offer no refunds). They use their most decrepit buses for this journey, but reserve their nicest buses for the North.
In Addis: Selam’s ticket station is across from Meskel Square down an alley. Sky Bus’s ticketing is done next to Taitu Hotel in Piazza. Both buses depart from Meskel Square. Selam only departs in the morning, but you may find that Sky Bus alternately departs at 1 pm in the afternoon (as it tries to do a round-trip journey to/from Jimma).
In Jimma: Selam’s ticket office is in same building as Central Jimma Hotel. Sky Bus office is next to Syf Hotel. Both buses depart from these ticket offices.
I could write in which days which bus travels between Addis and Jimma and vice versa, but that information will be completely obsolete by tomorrow. I’m dead serious. Your best bet is to get to the ticket offices two days in advance and check both Sky and Selam. My guess is that these luxury buses don’t understand the first thing about customer service: that you stick to a schedule and maintain reliability and consistency over the loss of a few birr here and there. Instead they would rather inconvenience you as much as possible.
I have to make the trip between Jimma and Addis quite often and will always try to avoid Selam and Sky bus if possible.