After purchased Sentenza Desktop, you have to get your serial number for license activation. This serial number (named Purchase Code) is in a text file that you can access in the Downloads tab (in CodeCanyon.net user panel or in email confirmation).
Screenshot (user panel) : http://www.sentenzadesktop.com/images/purchase-code-1.png
Screenshot (email) : http://www.sentenzadesktop.com/images/purchase-code-2.png

The username to enter is mine (seller) on CodeCanyon : ilovemypony

Now open Sentenza Desktop and go in License tab to enter your serial number, then Validate. Cool, now you are a registered user If you have some problems at this step, feel free to contact me (via CodeCanyon comment section).
Screenshot : http://www.sentenzadesktop.com/images/register.png

If you have some problems with .PKG package installer, use this method (screenshot) : http://is.gd/6IlVGs
Official Apple help : https://developer.apple.com/library/.../SubmittingYourApp.html