After grouping related tests together in a Smart Folder, you can run all the tests in a folder as you would a single test. To do this, in the Test tab, click on your "Smart Folder" view to view your tests as grouped into smart folders. Next click "Run Test" next to any one folder, make any adjustments to the run conditions and then you're set.
When triggering a run of a smart folder, each test in the smart folder will run against the platforms it is set to run against in the Test Edit view. If you opt to run a smart folder against a different set of platforms, you can do so by overriding the platforms in the Run dialogue.