I installed it as described. I got an error from the AutoLoader, so I patched the following 2 file references in referenceselector.php:
$wgAutoloadClasses['ReferenceSelectorExt'] = 'extensions\referenceselector\ReferenceSelectorExt.php'; $wgAutoloadClasses['ReferenceSelectorPage'] = 'extensions\referenceselector\ReferenceSelectorPage.php';
Still no button appeared...