In order to post updates through the Flickr API you'll need first to create a Flickr App here:
Click on 'Apply for a non-commercial key' button.
Once you've created your Flickr App copy your 'Consumer key' and 'Secret key'.
This is it, with 'Consumer key' and 'Secret key'
You can add your account in the Social Accounts section of MicroblogPoster.