After purchasing Song Master, you will be able to unlock the Song Master version you already have installed. You do not need to download and install Song Master again unless you want to. 

Display the authorization dialog box

If Song Master is currently running, you can get to the authorization dialog from the Help menu:

You can also click the Unlock... button when Song Master is first started:

 Authorization dialog

In the Authorization dialog, enter your email address and password that you used to purchase Song Master and click Register:

Once the email address and password are verfied, Song Master will be unlocked!