To create Mp3 files with Overtone Analyzer, the LAME Encoder (lame_enc.dll) needs to be installed on your computer. Since Mp3 is a patented technology that requires a very expensive license, we cannot distribute this file ourselves and recommend the use of the Ogg file format for creating compressed audio files. Ogg is a free alternative to Mp3 that also claims to have a better sound quality than Mp3 for compressed files of similar size (Wikipedia).
If you want to create Mp3 files, there are a number of sites where you can freely download the LAME Encoder.
For example: www.rarewares.org (The current stable version is LAME 3.98)