When you select "Install for everyone on my machine", the installed will properly copy the file and register the add-ins with Microsoft Excel, but it leaves them disabled (i.e. un-checked). This is consistent with Excel Solver or Analysis Pack Add-in.
To enable NumXL, each user must enable the add-in manually using Excel add-ins manager. There are two entries listed: NumXL and NumXL Toolbar. You should check both entries and click OK.
(Note: Install for everyone is only available for NumXL versions 1.63 and earlier.)