I noticed a different behaviour of OnCLickCheck event between win and osx.
In win I can do the following, Since ItemIndex is updated before
ClickCheck event:
procedure TForm1.CheckListBox1ClickCheck(Sender: TObject);
begin
   label1.caption:= inttostr(CheckListBox1.ItemIndex);
end;

while on OSX the ItemIndex I got in OnClickCheck is not updated. If I have not clicked on any item before ItemIndex is -1.

On OSX I have laz 1.6.4 and fpc 3.0.2.
I noticed a similar thread in 2009:
http://lists.lazarus.freepascal.org/pipermail/lazarus/2009-October/045577.html