Sign up in the sandbox here: https://www.sandbox.paypal.com/us/cgi-bin/?cmd=_product-go&product=premium_services
(for the curious, I got this URL by going to paypal.com/erp and then adding "sandbox." in front of the resulting URL's paypal.com)
Log in with your test account email/password (something like aaaaaa_############_biz@emaildomain.com )
If it says "Please login to use the PayPal Sandbox features.", that's an issue with the new sandbox that's being worked on--for now, clear all your paypal.com cookies , log in to developer.paypal.com again, and then visit the above URL again