Here is the link for the test: https://www.indiana.edu/~istd/test.html.
Remember to work through the examples: https://www.indiana.edu/~istd/examples.html. They have 5 examples for word-for-word plagiarism and 5 for paraphrase plagiarism. Go through at least one or two each in depth before taking the tests. You don't have to review all 5 of each unless you're struggling to understand.
If you have taken the test too many times and they shut you out, clear your browser history and you'll look like a new visitor (make sure you're clearing the cookies too--that's what the site uses to identify your computer). When you complete it, please email the certificate to me. Make sure your name is associated with it.
Plagiarism.org also has explanations if you're having trouble getting it from the Indiana University site. They also have a nice, fairly concise explanation of how to prevent plagiarism in your essays: http://www.plagiarism.org/plagiarism-101/prevention/.