Hi Pedr0_Neto,
Congratulations on the purchase of your new MacBook Air! I understand you are having trouble getting the Keynote application on your computer. Is this your first Mac, or did you set this MacBook Air up using a Time Machine Backup from a previous computer? If you have a Time Machine backup, you can restore the Keynote application and any presentations you had created in the past to your new computer, then update the software to the latest version, which is 6.5.3.
Is it possible that you are using a different Apple ID than the one initially used to purchase Keynote? If you have more than one Apple ID, you can sign out of the iTunes Store, then sign in with the other Apple ID and look at the Purchases.
I'll include some resources below that my help with this. If you are still unable to get the Keynote application to install on your new Mac, I'll include a link to technical support, as you are entitled to 90 days of support with your new purchase.
OS X Yosemite: Restore items backed up with Time Machine
Sign in with a different Apple ID on your iPhone, iPad, or iPod touch - Apple Support
MacBook Air - Contact Support - Apple Support
Regards,