If you are using any version of Tapjoy that is greater then 10.x the Tapjoy SDK Key can be found by navigating to Settings/App Settings from your main Tapjoy Dashboard in your app. Keep in mind each app/platform combination has its own SDK Key. Here is a screenshot.
If you are using a version of Tapjoy that is 10.x or less, you will have to contact support to obtain the "App ID:" and the "App Secret Key". We strongly recommend that you update to the most recent version of the Tapjoy SDK, as Tapjoy SDK 10.x is no longer maintained and may not be compatible with the most recent versions of iOS and Android.