In my standard MAME (exactly as in git, no modifications), if I go to the plugins menu, it is empty. Further, if plugin.ini was populated, it gets emptied out leaving a file of 3 bytes.

What have I done wrong?

Plugins are enumerated by looking for plugin.json in subdirs of pluginspath. Also, noplugins can't be set although that probably isn't you problem as the plugin menu disappears in that case.

Yes, that was it. For some reason my pluginspath was pointing to a non-existing folder. Changing it back to "plugins" fixed it.

Thanks!

Sorry to bring this up again..., BUT.... although it works fine from the main menu, if I start a game, press Tab, and choose the Plugin Options, the only option is to "Return to Previous Menu".

What have I done wrong?

Plugin options lets you configure the plugins which are enabled and have a menu (the cheat, cheatfind, timer and dummy plugins are the ones that have menus currently).

Ah, ok. Thanks!

Firstly tried enabling dummy then choosing a game, no luck.

Then I found that after enabling one or more of the plugins you mentioned, you have to quit, then start MAME again, and only then will it work.