Go to ShipSim.com, log in, go 'my account' click 'orders and downloads' all the products you bought will be there, just below it will say downloads, SSE should be a blue link, click it and the download will start.
Once downloaded copy paste your licence key from the licence key overview (just below the download overview)
Dont forget to download the updates for SSE, next to the shipsim logo, Home- Products-click Downloads¬updates
Hope that gets you sorted!