Computer repaired, was not given an account password

My computer was just repaired by Apple, and they reinstalled the operating system. The computer starts up without needing the password, but downloading applications, or a flash player is a problem. When I tried to download an application it asked for the account name and password. The account name was "Apple" but I don't know what the password is. And I can't change the password without knowing what the previous one was. I would like to not have to go back to the apple store to fix this. If what I said wasn't clear, I included a screen shot.