Add-ins are files just like Excel workbooks - indeed they are normally created from Excel workbooks. They can be opened and 'run' in the same way as any normal Excel workbook. Since they contain vba program code, the user will frequently be greeted with a Microsoft Office message like.

While you should be careful about the add-ins you run on your computer, there is no point in opening an add-in unless you are prepared to 'Enable Macros'. It won't work otherwise.
Excel has a special method for loading add-ins whenever it is initiated.
Add-in files do not contain any visible worksheets and in general you are not permitted access to the macros they contain from the 'Macros' button on the standard 'Tools' Menu.
For each add-in you load you must identify the method that the programmer has provided for using it. It could be from a special menu (e.g. the ACBA Functions menu) or from a special tool bar (e.g. the Audit Tools Shortcuts). As a rule, the instructions or user manual that comes with the software will tell you how to use it.