Before you can install SDonate, you will need to get your SDonate API key. To do this, head over to https://sdonate.com and sign in using your Steam account. Once you've signed in, click on "Account" at the top right, then see the section titled "SDonate API Key". Click on "Get API Key" to get your API Key, if you have purchased SDonate it will generate your API Key. You will need to use this key later while installing SDonate.
Warning
It may take a few minutes for SDonate to register your purchase from ScriptFodder. If it says that it your purchase was not found try again in a few minutes.