It's free! 

1) Download the Acquia dev desktop 2 from: https://www.acquia.com/downloads and install it.

2) As part of the install process you will be able to select openatrium. It should then install openatrium for you. You will then go through the standard install process, but it will have set up the database for you. If the quick method doesn't work, try the standard install method. If there are any problems then you can reply to this message and I'll have a go at working it out. Or you could do a search and see if you can work it out yourself.

God Bless