Alpha/beta access is Early Access, and has already started. See:
Kickstarter Update #29 - Play Divinity: Original Sin 2 RIGHT NOWBasically, you need to create an account on the
Larian Vault website, if you do not have one already, or log in, then link your Kickstarter pledge so you can request a D:OS 2 key.
For the Kickstarter email, check the Notifications page in your Kickstarter settings. Under the Newsletters (that you can subscribe to) is a section for backed projects, where you can enable or disable Kickstarter email for updates.
If that is enabled, check your spam folder, if applicable.