Find and replace is a Contact Sheet operation, it Finds and replaces text in the metadata of images in the Contact Sheet, it is not meant to operate on IPTC lists, so the fact that it is greyed out is by design.
The ability to do this within Photo Mechanic would need to be a feature we add at a later date, however, it would be pretty easy to accomplish with the Import and Export options and a text editor.
With nothing selected in the list, press the 'Export' button to export the entire list to a text file. Then, open the text file in a text editor and do your find and replace and save the file. Then, use the Import button and import the file that you saved. You will first want to clear out the current list, because when you import the text file, it will just add items to the list that are not already in there (anything that would have been modified by the Find and Replace).
Hope that helps.
