Can't add the buttons to script.

Simply,I made a level controller script, to my level menu.And I made a gameobject as MainMenuController and added the script into it.I can add buttons to script in gameobject but when I get script alonel I can;t add any buttons.174242-buttonerror1.png

In here u can see something tell Default refernces wii only be applied in edit mode.what is this?Please help!

For Scripts you can only add Pfefabs as public Variables, because Unity don’t know in wich scene you will use this script.