Forum Moderators: open
Welcome to Webmasterworld.
I hope I'm correclty understanding your question. If so, here's an answer.
The "Find Next" button is only enabled after a search string is entered in the "Find What" field. The "Find in HTML" checkbox is only enabled if the "Find where > All pages" radio button is checked.
Depending on the size of your web and your machine's memory FP can be quite clunky in a find and replace so be patient. And DON'T initially do a "Replace All", check your search and replace strings with a simple Find Next and Replace a couple of times to be sure FP didn't change any of the expected code.