Martin..
To use the youtube and flickr API, a developer key is needed. I´ve registred myself in the following sites.
http://www.flickr.com/services/api/
http://www.youtube.com/dev
I thinks it´s probably a good idea for you to register and get a key specifically for moodle.
Then... just change the API keys in the block_tag_flickr.php and block_tag_youtube.php