Right-click the palette section header "yed MyUML" to open its context menu. Choose "Convert to Document". In that document, select the element for which you want to specify a tool tip. Enter the desired tool tip text in field "Palette ToolTip" in the properties view in yEd's lower right corner. Once you have specified all tool tip texts this way, open the palette section context menu once again and choose "Adopt from Document".