If you pre-ordered from the Larian Vault, send an email to support@larian.com.
However, balance and game speed will be adjusted as the beta progresses. You can set the enemy AI to 'None' to get a feel for the game mechanics and controls without any time constraints.
EDIT: The next beta will by default have a slower game pace, and include other enhancements (see
here).
There are a few gameplay videos, etc, that could help give you an idea of what to do, and some tips scatter about,
like here or your other topic.
Let's Try Divinity: Dragon Commander - Single Player Campaign (20 min)
Also, like Watser said, in the single player campaign modes you can auto-resolve battles, either sending in units alone or with a general of your choice (each have different strengths and weaknesses and will influence the chance of winning particular battles).
