After is mounted and installed to register Stellar Phoenix Mac Data Recovery software please introduce the licence key you've bought: 

Click on Register button on the main user interface or select Register option from the Tools menu.

Click Tools than Register

Now registration window is displayed. Select ‘I have the key to register the product’ and click on OK.

If licence key has been bought, select the option and confirm with OK

Last step: Copy and Paste your key in the Registration Key text box and click on OK.

Copy and Paste the licence key then press Register

In case you need assistance with Stellar Phoenix software, please contact the dedicated support team of Stellar Phoenix.