Re: [Mac] - Sims 3 won't start after i press play on the launcher
The simple answer is that your Mac just can't handle all those EPs installed. The Sims 3 is a very intense, resource hungry game, especially on Macs due to EA having used Cider to make it 'compatible' in OS X. Pets and Seasons are especially demanding. Showtime isn't much better either.
MacBook Airs are not designed for gaming, they are not really powerful enough. And whilst you may be able to play the base game on all low settings, with memories turned off, when you start adding in EPs and/or up your graphics settings the game will just give up.
You should also update Yosemite to 10.10.5, the latest release.