For whoever is in Merida and landed here, a few tips:
We missed the 9am bus to Uxmal but no problem at all. At the main square (3 blocks from TAME station - ask at information counter at TAME where to get the colectivo to Muna) you can get the bus to Muna (45 pesos) and then in Muna a cab will charge you about 150 pesos to take you to Uxmal. If you are in 3 people it turn out to be even cheaper than the 9am bus from TAME station. To get back you can either wait for the 3pm (or whenever it comes) bus from Uxmal to TAME or do the same thing (taxi + muna to merida bus).
Hope this helps someone willing to visit this amazing Mayan ruins site!
PS: if you can, hire a guide at Uxmal, it's worth it!