STORM for MSFS will release on the in-sim Marketplace in the future and is currently not available yet.
We will be submitting it for approval and will release on the in-sim marketplace in the near future.
Once it is available, you can purchase by searching for STORM on the in-sim marketplace. From there, open the product page and click DOWNLOAD AND GET. This will prompt you for payment. After payment has cleared, STORM will automatically download to your simulator.