Once theme has been successfully installed (and you have logged out and back in again), you will need to navigate to the System > Design screen. 

Once on the Design screen, click 'Add Design Change', and in the General Settings area, nominate the Store view and then choose the theme from the 'Custom Design' drop down. 
*There are the required elements, however you can also set a start / expiry date for the theme to be active for. Once configured, click 'Save'.