If you have redeemed your license key and the launcher has failed to automatically download you can manually complete the installation using either the Windows Store or Xbox App. Follow the steps below for your preferred application.
Complete installation via Windows Store
1) In the Windows search bar, search and open the "Microsoft Store" app.
2) In the bottom left corner, click "Library".
3) In the top right click "Get updates". Wait for all updates to complete before moving to step 4.
4) On the same page, scroll down until you see "Microsoft Flight Simulator". Depending on the edition you own this may be named "Microsoft Flight Simulator", "Microsoft Flight Simulator - Preorder", or "Microsoft Flight Simulator: Premium Deluxe - Preorder". Once found, click the download button.
Microsoft Flight Simulator will now proceed to download and install. Once complete, you can launch the simulator either on the same store page or by searching "Microsoft Flight Simulator" in the Windows search bar.
Complete installation via Xbox App
2) Click the Microsoft Flight Simulator icon on the left sidebar.
3) Click the "Install", "Play" or "Update" button. If an install or download is required you will be able to click the launch button after completion.