So what is an add-in? They are like the plug-ins that go with Adobe, etc. They are actually Access databases written by a third party. The extension is typically MDA. You install them via the Add-in Manager under Tools | Add-ins. Once installed they are listed under that menu.
There are several companies you can find on the internet that sell these products. One that we use allows one to export or import multiple objects at the same time. It comes in handy when we are upgrading a client. Another one a client uses provides fancy Excel import/export capabilities.
Of course there are many more such addins. Do you have one you especially like? Let us know.