What’s the betting it can’t find rfm_send( ) either?
You need to extract the DIRECTORY “emonTx_3Phase_PLL” that was in the .zip file you downloaded, and put it complete with all its contents into your Arduino Sketchbook directory. I’m fairly certain it will then compile when you use the proper Arduino IDE. I can’t speak for anything else.
(If in doubt, use the checksums in the post to verify the integrity of the download and the constituent files after you’ve unzipped them.)