Usually, the registration is processed and approved within 1-2 days.
ATLAS / lxplus
The ATLAS registration is available from
http://atlassec.web.cern.ch/atlassec/Registration.htm
Choose the form in the section "external participation". Provide the completed form to Anja Schielke, who will take care of the signature by Siegfried Bethke and sending it to the ATLAS secretary. As the registation process takes a few days, better start the process in advance.
Setup
Up-to-date instruction how to setup the most essential software on your local machine, the Rechenzentrum Garching (RZG) and the CERN computing cluster (lxplus) can be found here:
https://twiki.cern.ch/twiki/bin/view/Main/MpiQuickStart