In order to view replay files (*.rpy) that people post in the forum areas, or that are included with setup files that can be downloaded elsewhere, save the .rpy files into your Documents / iRacing/ replay folder.
Then, from the iRacing UI click Replays
This page will list all of your replays and then click the green Launch button to launch the desired replay:
If you are still using the members website via a browser, go to members.iracing.com and go to Results & Stats...My Replays:
Then, on the replays tab, you have the choice of either Online Replays or Testing Replays:
and then click on the File Name of the replay you want to launch.