Take a look at the FAQ page. It has the info listed there: http://www.dualthegame.com/faq
But to summarize, you need to be a Gold level founder or higher to have access to the alpha. Please note that according to a release on July 5th, they will be removing the option to become a "founder" on September 7th, 2017 for a complete overhaul of the crowdfunding campaign. In doing so you will no longer be able to obtain the ability to access the first alpha version (Alpha 1) if you have not already purchased a Kickstarter pack or a Founder pack. However purchasing a "Supporter Pack" which will come out later in the year will add the option to catch the future revisions of the Alpha.
From https://board.dualthegame.com/index.php?/topic/11308-end-of-the-founder-packs-crowdfunding-portal-evolution-alpha-news/
