I think they mean to uncheck "Volume Data" and set the "Volume Effect" drop-down to "None" in the Find tab.
For the case in your screenshots, you would want to do the above, and then in the Replace tab, select "v: Set Volume" in the "Volume Effect" drop-down and set the "Volume Data" to your desired value. Keep in mind that this will also write a volume command in completely empty pattern rows if you don't specify something else to search for (like a note or instrument).
Alternatively, though it doesn't have the searching flexibility of Find / Replace, you could use the Amplify action (accessible through the context menu or, with default keybindings, Ctrl+M) on a selection you want to change the volume of.