In REAPER, just select File -> Render, and render as an mp3 (you may be missing a plugin, which REAPER will direct you to download, and give you the proper link). You shouldn't ever need external software to save a song out.
render as a .wav file (reaper can do this) and note where you saved it to. open switch and click the add file button at the top left. at the bottom left, make sure mp3 is selected on output format. select your output destination (above format) and click convert.