March 2015 update: The menu style interface has been superseded by the Office style ribbon interface. See this article for more information.
While earlier versions of Microsoft Excel allowed a developer to add their own menu to the menu bar relatively easily, the newer versions have thrown a bit of a spanner in the works when it comes to creating custom menus. It does not surprise me then that users often ask "where do I find Ian's Excel Addin after I have installed it? Fortunately, the answer to that is relatively simple.
After you have installed the addin, you will find the menu under the Add-Ins tab in Excel. The image below illustrates this in Excel 2010.
Remember though that not all of the addin's features are available from the menu. One of its most powerful functions, textamount, is an Excel function that is used in formulae. See this article for more info.