Morelia has an international airport (airport code MLM), but there are not many direct flights to Morelia from outside the country (although there are some).
Continental has a direct flight to Morelia from Houston (CO 2694 at 5:40pm) and Mexicana has a direct flight to Morelia from Chicago (Mexicana 175 at 8:45pm).
Most other air routes to Morelia will go through Mexico City.
Alternatively, you can consider flying into either Mexico City or Guadalajara and taking a bus to Morelia.
In each Mexican airport there is a pre-paid taxi booth where you can buy a taxi ticket to the Central de Autobuses (in Guadalajara) or Central Poniente, also called Central de Observatorio (in Mexico City). The fare is about 200 pesos. From the Centrals in both Guadalajara and Mexico City there are frequent (very comfortable) buses to Morelia with one of the companies Autovias or ETN (at least one per hour from 6:00am to 8:00pm, slightly less frequent between 8:00pm and midnight and none after midnight until morning). The bus ride costs about 300 pesos and takes between 3 1/2 to 4 hours without stopping. The buses are equipped with air conditioning, TV, VCR's, etc. If you advise us about your approximate arrival time we will make preparations to wait for you at the bus station or airport in Morelia and take you to your hotel.