https://bugs.winehq.org/show_bug.cgi?id=53161
--- Comment #3 from Hans Leidekker hans@meelstraat.net --- (In reply to Melroy from comment #1)
So winemenubuilder.exe is indeed not located in the syswow64 folder. But only in system32.
I believe winemenubuilder.exe is 64-bit only on purpose. There's no need to have both a 32-bit and 64-bit version because it's an internal utility.
You should start it like this:
$ wine64 winemenubuilder