How can I setup a site with Acquia Dev Desktop 2 beta?
To do this, I used dev desktop to create the site instance (instantiating the database and vhost files within Apache) than overwrote that with the install I wanted. Specifically, 

I created my site instance normally, selecting "Acquia Drupal" or any of the installations when it asked me to choose a distribution. 

For my site settings, I changed it from the long name to 'drupal8' for simplicity

then I went to my local sites directory through terminal and removed the Drupal 8 installation it created (these commands assume OS X environment)

cd ~/Sites/Dev\ Desktop/
rm -rf ./drupal8


Now I can add whatever I like back into this directory, making sure I create it with the same name:

git clone --branch 8.x http://git.drupal.org/project/drupal.git ./drupal8


Now we should be able to install this normally with a couple changes at the install stage. For the database settings, you can use 

username: drupaluser
password: &lt;blank&gt;


as Dev Desktop uses these credentials for all the databases. 

Under advanced configuration there were two other settings you need to get this to work:

port: 33067
localhost: 127.0.0.1