Su.pr comes with your StumbleUpon, so you can log in with the same username and password as you do for your regular StumbleUpon account.
Su.pr is a publisher tool that allows you to post your own content or any sites you like to your Twitter and Facebook fanpage.
After you log in to Su.pr, go to Settings, connect to Facebook and add the pages that you manage.
Then go back to the Su.pr posting interface and post a link. Make sure the page you want to post to is checked on the sidebar.
We've been using this successfully for our own facebook page:
http://www.facebook.com/StumbleUpon - you can check out how the posted links look like.
Hope this helped. Please let me know how you like this feature and if you need help with anything else.