After completing a find or a replace, you can press Command-G to find or replace again. Z-Write will continue the search from the cursor's current position.
If you choose "Replace All" from within the Find/Replace dialog, Z-Write will replace every occurrence found in the Section or entire document. Note the undo for "Replace All," only works within the current Section, so make sure you really mean it! (As a safety precaution, you can save just before doing a Replace All for All Sections and do a Revert if the result isn't what you expected.) If you interrupt a Replace All (by pressing Command-period) Z-Write will stop replacing wherever it is, leaving some occurrences replaced and others unchanged.
You can also choose the "Find" button on the Toolbar. Holding down the Option key while clicking the button acts as a "Find Again" (Command-G) command.