Unzip and upload it to your bb-templates folder, login as admin and go to Appearance, there you will see all your uploaded themes, click to activate.
Instead of uploading to bb-templates folder, create a folder my-templates in the bbPress root and upload the theme to my-templates folder.
Then you can activate it under the Appearance options.
If you face issues, then fix out their permisison issue by following this tutorial : http://blog.ashfame.com/2009/09/fix-my-templates-my-plugins-folder-not-working-bbpress/
Do not upload to bb-templates of bb-plugins. Make two new folders, if they’re not already there, called my-templates and my-plugins, and put your unzipped templates and plugins in those folders. There is no upload in the admin for themes or plugins, like there is with WordPress.
bbPress for a while was coming with those two folders, empty, but with the wrong permissions. Follow the link from ashfame to see how to fix the permissions issue.