I’m trying to modify the Eurolatin keyboard, so I need to know where it’s installed. The tutorial I’m using seems to assume that they’re in \Documents\Keyman\ but I don’t have a Keyman folder in Documents.
Keyboards you download and install are not in a version you can modify with Keyman developer. To modify a keyboard you need to look for its source. The source package for the EuroLatin keyboard is at https://github.com/keymanapp/keyboards/tree/master/release/sil/sil_euro_latin
In that package, open the source folder, and the desktop keyboard file is sil_euro_latin.kmn and the touch keyboard layout is in the sil_euro_latin.keyman-touch-layout file.
To add to what Steve wrote, once you have downloaded the source files – everything in the sil_euro_latin folder – the best place to start is by opening the project file in Keyman Developer – in this case, sil_euro_latin.kpj.