If you don't remember registering, you probably aren't getting it. First you should check your PCS website. It should have all the application and test data for the law exam, with all the check boxes. If all you see is the PE, then you aren't registered.
They offer the law exam almost all the time for those getting a NJ license that already have one in another state. You need to register for it and pay the $20 or whatever fee is these days. If you are taking it in conjunction with another exam, PCS will mail out the test and you can download the reference material from the NJ Board website. PCS usually gives 3 to 4 weeks to receive it, complete it and mail it back. I had to take it for the Surveying exam and I got it in mid March and it needed to be returned April18th. If you fail it, or don't return the test by the deadline, the only option is to take it are at a testing center. Since you passed the PE, once you receive the paperwork, or email from the NJ Board congratulations, they should have directions for how to set it up.
To give you some idea of how easy it is...I finished it about 15 minutes and got a 95%. I should have gotten a 100%, but all you need is a 70% I think.