Starting with version 1.8.x (I think) the first time you start SourceTree you get a "activation" splash screen. Here I accept the license agreement, then I move on to "Atlassian account" where I'm asked to use and existing account or got to My Atlassian for signing up. There is no option to apply a downloaded .license file.
How can I use SourceTree on a computer that's not connected to the internet?