Accounts for PPX and PopSci.com are tied together into a single login. This way, as well as playing PPX, you can also comment on, rate, save and share all your favorite PopSci articles.
If you already have an account on PopSci.com, log in and click the "Your Profile" link below your name to the left of the PopSci.com logo. After clicking the "Edit" tab, check the box marked "Please register me for a FREE account on the PopSci Predictions Exchange" and then click the "Submit" button at the bottom of the page.
If you don't have a PopSci.com account, simply create one at popsci.com/register, making sure to check the same "Register me for a PPX account" box.