In order to get your API keys, you should register in the system at http://www.tumblr.com/oauth/apps or login to the system if you are already registered.
After successful authorization if you have already had the applications, you will see the secret keys on this page.
If you do not have these keys, you need to register a new application. In order to do this you should go to https://www.tumblr.com/oauth/register and fill in the corresponding application form.
After successful registration you will see your API keys.
The Application API and Secret Key should be entered in the corresponding fields of the extension configuration for Tumblr in the admin panel of Magento®.