1. open UnrarX
2. drag the .rar to UnrarX (in the Dock or the application window) or browse via the Browse button/text
3. press the Password button/text
4. enter password and hit enter
5. press the Extract button/text
There is also The Unarchiver, but I don't know about its password abilities.
unrarx tells me to hit the upper right bullet which will bring up a password request. havent tried it but maybe it works. hardly user friendly ... on the other hand it's free.