Ethiopia is largely a cash based society. Credit cards are accepted at only a very few outlets in Addis Ababa and it is not normally possible to get currency advances against a credit card. International bank and debit cards are accepted at major banks’ ATMs in the capital but there are very limited banking facilities in most other areas. Make sure you have an adequate supply of cash before travelling outside the capital.
If you change foreign currency into Birr, you should ensure that you keep your receipt, as without this you are unlikely to be able to change any left over money back at the end of your trip.