Excel loads the add-in in two parts, the Taskpane and the Custom Functions. Getting an _xldudf_ in your formula indicates Excel is not able to link the names to the custom functions.  Most commonly, the source of this problem is the custom functions portion is not loaded correctly. There are a couple of troubleshooting steps to try when this happens: Firstly, load up a new, empty document, login to Scott’s Add-ins and see if you can run a custom function. If that works, then go back to your sheet and manually change _xldudf_SCOTT_DESC to SCOTT.DESC. Excel should do this automatically, but it appears it is not. If you cannot run a custom function in a new sheet, then the installation is not working.  We advise removing the add-in, restarting Excel, then re-installing the add-in.  This should clear out any caching issues, and hopefully get you working again.