I created a shortcut on the desktop to Reaper and when I tried to open Reaper I got the window that says I'm still on an evaluation version, though I had already entered the license info under the admin user back in April. I had to set up another user without admin priveleges on the DAW. I had done this before when I only had the admin user on my DAW. All I know is that the email says to copy the license from the email and then import into Reaper. Forgive me as I'm not at my DAW right now so I can't give you exactly what it says. The body of the email has a very long stringed license but there is also and attachment to the email that says Reaper License.rk or some such thing. I thought you'd received a serial number.
No, if you received a license file as an attachment to your mail that's the file REAPER is asking you to look for.