Define in your .htconfig.php:
$a->config['tumblr']['consumer_key'] = "your-consumer-key";
$a->config['tumblr']['consumer_secret'] = "your-consumer-secret";

You can get it here:
http://www.tumblr.com/oauth/apps

Tumblr-OAuth-Library:
https://groups.google.com/d/msg/tumblr-api/g6SeIBWvsnE/gnWqT9jFSlEJ