You can use US dollars, Myanmar Kyats, and Foreign Exchange Certificate (FEC).
FEC can be used everywhere in Myanmar and is the same rate as US dollars. However, you can fetch a slightly better rate with money changer in Yangon when buying Kyats with FEC than US dollars. You can alos change the FEC back into US dollars.
Bring cash in US dollars and change them to FEC or Kyats when you arrive in Myanmar. Credit cards and ATM machines are rarely available.