I think you need to wait till the official ARR service is started (near launch) before you can register your code. But, that's in case you haven't registered your 1.0 copy back when 1.0 was still online. If you've registered during that period, just downloading the client from the beta forums should suffice.

If you don't have access to the beta forums yet, please wait till the 4th Beta Phase is about to start and check your mail regularly for a Beta Key (if you need one), including your spam mail box. If by that time you still don't have access to the beta forums (and thus the launcher), please contact the SE ARR Support.