I don't know which buttons I have to click or what key combination to type to calculate a cube root.
The nth root is calculated when you put a subscript number before the square root sign.
₃√27
To enter subscript numbers with the keyboard hold down Alt while entering the number.
E.g.
 Go to Advanced mode
 Press Alt while typing 3
 Press √
and so on


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1@CristianMirTienda Were you satisfied with my answer? Then give me a upvote (∧). If I could solve your problem, then it would be nice if you'd mark my answer (✓). – A.B. Apr 18 '15 at 14:36

Open Calculator and goto Help>Contents (or simply press F1), You will find help for Powers and Roots:
Where you will find information that:
nth roots can be calculated by putting a subscript number before the root sign.
₃√2
The superscripting and subscripting numbers can be put by holing Ctrl and Alt respectively!.