Hi Ethel,
Try using the following code instead:
// Remove default Thematic actions
function remove_thematic_actions() {
remove_action('thematic_header','thematic_access',9);
}
add_action('init','remove_thematic_actions');
You're trying to access the wp_page_menu directly. This is already part of the hook. So, you need to remove the hook action instead.
Also, you have the string "optionalpostitionnumber" for the position number argument. This was just a place holder to tell you what type of value you can use here. It's actually a number. In my case, it's 9. It should be in yours too. Look in hooks-filters.php in the Thematic theme folder under "library/extensions/" directory. You should have a line that looks like this add_action('thematic_header','thematic_access',9);
The 9 represents the position of the hook when it was added. But do not remove that code. Just remove the action reference to it in your function.php file as you started to do already.
Hope this helps.
Matt