I recently used one of the MPT (Myanma Posts and Telecommunications) 20,000 kyat GSM SIM cards without any problems.
No activation was necessary - simply put the card in your phone and it works.
The card will have a phone number on it and for people to call you from within Myanmar you need to add 09 to the beginning of the number on the card. For people to call you from overseas you need to add the country code 95 followed by 9 to the beginning of the number on the card.
The only set up needed on your phone is if you want to send SMS messages - you will need to change the SMS Message Centre number in your SMS settings on your phone to +9595930003.
SMS cost is 5 cents a message but you can only send SMS messages to phones within Myanmar.
For balance check you dial *124#.
It worked well in each of the places I visited - Yangon, Mandalay, Lashio, Pyin Oo Lwin, Bagan and Inlay.
You cannot top up the card. If you run out of credit you need to get a new card with new number.