Okay, got it in the end. Tags, notes, etc. working. I've been bumping into the problem of “selective documentation”, so common with open source and free tools: a big chunk of the syntax or actions are omitted because the developer does not realise that some things are not obvious.
Many plugins for DokuWiki are automagically installed by the wonderful Plugin Manager, but some developers omit telling you that the bit of code they list (often in another part of the documentation, outside the “Installation” paragraph) must be embedded in the main.php template… aaanyway.