You don't apply to Sloan. You enter MIT with all other freshmen, take the same General Institute Requirements
courses (GIRs, the MIT-equivalent of core courses), and then starting as early as spring of your freshman year you declare your major and sign up for courses accordingly. This could be a major in the Sloan School, or something else entirely. You don't apply for your major (with the exception of the new Biological Engineering major
which has limited enrollment by application in sophomore year). (Right, mollie?)
Your statistics and activities will likely put you in the competitive pool, after which your chances, like everyone else's chances, are roughly 14 out of 100, if last year's ratios hold.