You can now get Office Add-ins from the Store or use Add-ins you already have from right within recent versions of Word for Mac and Excel for Mac.
There are two kinds of add-ins: Office Add-ins from the Office Store (which use web technologies like HTML, CSS and JavaScript) and add-ins made by using Visual Basic for Applications (VBA).
If you're looking for a built-in add-in such as Solver or Analysis ToolPak, select the Tools menu and then select Add-ins.
Office Add-ins aren't available in Office for Mac 2011. They're only available in newer versions.