Go to
this site and click the "DSST Practice Exam" link. That will take you to the sign-in form. It's the same form for real DSST exams and practice exams, but (and yes, this is a little strange) you have to have two separate accounts for real exams and practice exams. It's probably not a bad idea to pre-register before you go to take your first exam, but it's not strictly necessary -- you can do everything on-site. You will need your credit card each time in any case, as it is not stored on the site. If you don't pre-register, make sure you do look up the four-digit code for the college you want your scores submitted to.
-Gary-