Basically your tumblr username/password won't suffice in order to post automatic updates, tumblr api requires some more steps described below. No worries, it's rather a simple procedure.
In order to post updates through the tumblr API you'll need first to create your own tumblr App here: http://www.tumblr.com/oauth/apps
Click on 'Register Application' button.
Once you've created your tumblr app copy your 'OAuth Consumer Key'.
Also click on 'Show secret key' and copy your 'Secret key'
This is it, with 'Consumer key' and 'Secret key'
You can add your account in the Social Accounts section of MicroblogPoster.