I then tried "Tools" => "BuildLazarus..." and that seemed to get much further than when I initiated it from the package install dialog. However, this rebuild eventually failed on the "DateTimePicker" component...
Warning: Recompiling DateTimePicker, checksum changed for /Applications/Lazarus/lcl/units/x86_64-darwin/lclintf.ppu
datetimepicker.pas(85,12) Error: Can't find unit DateTimePicker used by DateTimeCtrls
So basically, I cannot, at present, install the TreeListView package or rebuild Lazarus as a separate operation.
This is not good! Can anyone shed some light on what the problem could be?