let's start with where to find it
With Inventor launched you will go to the "Tool" panel on your ribbon bar and then select the exchange app manager.
Once you find it and get direct to the page to download it you will just have to pic whether you have win 32/64 and the language and then click on the free button to download it.
Once downloaded; close down Inventor and run the installation. When the installation is done launch Inventor again.
To test it out start a new part with a cylinder; for mine I created a 1/2" diameter cylinder at 2" long and added the thread feature to the cylinder.
Go to the "Tools" panel again and there you will find the Thread Modeler button. Pick the button to run the command and then pick the thread feature from the browser bar (can't pick the actual part).
The program inputs the information it needs and all that is left is for you to click the "OK" button for it to do its thing. Below is my finished product.