Add-Ins can be loaded by 'Browsing' to the file or can be auto-loaded by means of an installer program written in another language (VB, C+, etc).).

Saving a workbook as an add-in, (File>Save As...>Add-In) and then loading it, by checking it's box here will also do the following:

1. Prevent the macro warning dialog from appearing.

2. The add-in workbook will not be visible.

3. You won't be asked to save changes in the add-in.

4. You won't be able to stop the code running by holding the shift key.

5. You will not see any macros in the add-in, in the macro dialog. (They can still be run, but will not be visible. You also will not need the workbook qualifier).

If you are receiving errors when starting Excel then this is where you should start to resolve the issue. Uncheck all add-ins an then gradually add them back, re-starting between each.