We can read this in the http://unity3d.com/support/documentation/Components/class-GuiText.html
When entering text into the Text property, you can create a line break by holding Alt and pressing Return.
alt+Enter dose a sound, but nothing show up that look like a break line.
You can use this to signify a line break:
\n
For example:
someText=“This is line 1.\nThis is line 2.”;
No.
If you mean entering text into Unity’s text field, the ‘alt-return’ shortcut seems to be broken on windows. You can get around this by copying and pasting a return character from some other text program such as notepad.