I chose attiny84 @ 8MHz board, and the following errors occurs when compiling:
CosaBlinkClassic.ino:35:29: error: Cosa/OutputPin.hh: No such file or directory
CosaBlinkClassic:37: error: 'OutputPin' does not name a type
CosaBlinkClassic.ino: In function 'void loop()':
CosaBlinkClassic:41: error: 'ledPin' was not declared in this scope
It seems that the included file is not found!