Go to the Add-Ins section in Excel Options, and in the Manage section, select Excel Add-ins and click Go. Browse for the add-in file, and when asked to save it to the add-ins folder, click OK. Once you are back to the add-ins list, just ensure that the check box is clicked, and press OK. Your add-in should now load every time Excel is loaded, whether or not any other workbook is.

For some other methods on how to Load and Add-in, try Loading an Excel Add-in