To elaborate: Use CTRL + H for indentation in mIRC's script editor, or use space-space-space-space in Notepad. Tab-indent (0x09) is not supported by the mSL parser.
For extra fun: the Tab character is used in Menus to signify a right-justified portion of the menu text, usually an accelerator hotkey. eg: Copy Nickname $+ $chr(9) $+ Ctrl+F7: /CF7
