Options: Preferences: Audio: Device should take you to the menu where you can select your focusrite, as I'm sure you've found. Don't know why it isn't showing up if you installed the drivers properly. You might try deleting and reinstalling the focusrite software just in case.
Once you've got Reaper recognizing your interface, you might just want to set up a project and try some test recordings. You'll see the button to set tempo and time signature bottom of the main screen, center. You'll need to set the input to a track in order to record. To do that, right click on the record arm button--the round red button to the left on track tab. You'll need to tell the track to take its input from the focusrite channel that you are using. There's also an option to set your monitoring--you'll want to set it to monitor unless you are monitoring through the focusrite's mixer.
Then press the big red record button and give it a try.