After installing or updating Retro Mix Legends in Studio One 5 on Windows, the plug-in does not appear in the Browser or the MixFX section of the Console. This is caused by a library incompatibility between the current version of Retro Mix Legends and Studio One 5 on Windows.
macOS users and users of Studio One 6 or later are not affected.
Solution:
Uninstall the current version of Retro Mix Legends via Studio One > Studio One Extensions.
Download the compatible version for Studio One 5 from your my.fender.com account here: https://my.fender.com/products/studio_one_mix_engine_effects. Select one of the plugins and you will find the installers in the Misc section.
Install the downloaded version manually by dropping the installer on to the start page.
Note: Do not update Retro Mix Legends through the Installation dialog after installing the compatible version, as this would re-install the incompatible version.