I don’t see anything wrong with your code, it must be that wp_nav_menu() is not called by the theme or no menu items are defined so the function exits prematurely.
To be thorough, you can verify your filter was properly registered by doing a var_dump() of global $wp_filter, or put a couple test die() statements in wp_nav_menu() to confirm it’s not being called.
I have come to the conclusion that messing with menus is simply not a plugin thing to do. The theme people and the wordpress core people seem to do things in such a way as to prevent isolation of ones code in a valid plugin. That may well have to do with an idealized view wherein “functionality” is to remain separated from “presentation”. The intersection of these two worlds happens at “menu”.
If I administer the theme properly, and name and select the menu “bar” appropriately then the plugin code can be made to work as I want. I think that is messy, but I will have to abide it.
I appreciate your taking an interest and I did what you said. The code that links up to the filter I was using is not executed because of the way the theme is done. That will be a problem (I think) with all themes. My reaction is to figure out a way to make my stuff work regardless of the theme. But that is probably not the best way to go. I need to “fit in” with the way the theme people do things. It means that I will have to provide good documentation on how to do the menus using the “appearance->menus” manual stuff….. Oh well….
