The full procedure :
1°) preorder the game
2°) when a new beta phase starts and you have preordered (beta 2 is supposed to start tomorrow), you receive an email with a serial.
If you preorder just after the start of a new beta Phase, you might still get in (I think that for beta 1 the time windows was one week after the new beta), else you will have to wait for Beta 3 later this summer.
3°) install Impulse, and register the game using this serial
(note: if you preordered using your impulse account, the serial might not be needed, but you still have to wait for a new beta phase to start)
4°) run impulse, with the setting "show beta/unreleased games" checked in order to have Elemental displayed in the "My Games" tab.
5°) Install the beta and enjoy the torture*. 
* it has been said that Beta 2 could maybe come with some fun included, breaking a long tradition of tedious beta-testing.