There are several ways to install your add-in. You can have the user open the add-in just like any other workbook. This is most appropriate in situations where you do not want or need to have your add-in installed in the AddIns collection (that is, dictator add-ins). The user will be presented with the virus protection dialog if they have virus protection turned on (the default). See the upcoming Macro Virus Protection section for details concerning the virus protection.
You can also have the user open a "loader" application (workbook) that in turn opens your add-in. This method can be used to run your add-in, as well. The loader can also use the Add method of the AddIns collection and the Installed property of the AddIn object to load your add-in using the Add-In Manager. Installation through the Add-In Manager is most suitable for committee add-ins.
A third option is to load your add-in into the AddIns collection directly by setting all the correct registry entries using the Win32 API. This is a highly sophisticated task and should be undertaken only with a thorough understanding of programmatic registry manipulation. Generally speaking, it's much simpler to have a workbook that handles the installation and calls the appropriate methods provided by Microsoft Excel.