To register your interest in attending log in with your CERN computer access account (log in button can be found in the upper right corner of the pages).
If you are in particular interested in an idea that is posted under the ideas section then click on the idea. You can express such interest by submitting a form with your skills under the idea description (more precisely you will find it on the page under the prerequisites).
You can register until the start of the event.
To register an idea or to register to be a mentor use the blue buttons "Submit your idea" and "Register as a mentor" on the welcome page. You can register your idea until the 30th of July.