If you are using TurboTax Online you will go into your account the same way as you did in the past and be able to see your past years and do your new return this year.
If you are using TurboTax CD/Download you will need to transfer your information with a key as seen in our TurboTax FAQ: How do I uninstall TurboTax from my computer?
Unfortunately, our TurboTax CD/Download is not compatible with the MAC computer so you will only be able to use our TurboTax Online software.
Thank you for choosing TurboTax.