To get n-Track (6.0 or later) to create an mp3 file simply select "File/Save As" and select .mp3 as the extension of the file to save to, from the "Save As type" extension drop-down box.
n-Track Studio uses a third party mp3 encoder called Lame Encoder. The program will prompt you to download the encoder on the first attempt to convert to an mp3 file. For instructions on how to install Lame Encoder please see n-Track can't find mp3 encoder even after I've installed it.
You can download the Lame Encoder dll from https://ntrack.com/redist/lame.php
Then
If you have previously installed a different version of Lame encoder that for some reason doesn't work correctly with n-Track, before following the procedure above close n-Track, remove the previous version of the Lame dll from the computer disk (or move it to a different, backup location), then follow the procedure above, making sure that n-Track asks you again to find the location of the Lame dll.
n-Track has been tested with Lame version 3.97 and later.
64 bit: Note that the 64 bit version of n-Track requires a 64 bit version of the Lame DLL, it will not accept/work with the regular 32 bit version of the Lame DLL