Welcome to the Magix Sound Forge community @Bill-Lonabarger and happy new year.
Was your PC on-line to register the encoder when installed and the MP3 encoder opened. Otherwise try a uninstall .. re-install. I recall this issue happening before, search this forum.
where is the .mp3 encoder? I was following the installation process for Audio Studio 13 when it prompted to enter the serial number, which i did. I did not see anything about an .mp3 encoder. why wouldn't it be part of the purchased software?
I found fix for this problem. Open sound forge – in upper left corner select “file” then select Reset all preferences and Cached data. Program will shut down and restart. Once restarted it will look completely different and the system recognizes .mp3 files.