I've never used that add-on, but I might suggest taking a look at my Social Feed addon. It supports Tumblr, and you can even display multiple accounts at the same time!
Your tumblr username should be just the name not the full url tumblr name. So if your username is myusername.tumblr.com than just put the myusername part in without the .tumblr.com part
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http://www.concrete5.org/marketplace/addons/social-feed/...
It's much more reliable and has lots of customization options.