Ok this is a file with a simple function to get cubic root of a number. The function is called cRoot().
So if you type =cRoot(A1) in any cell, it will display the cubic root of whatever value there is in cell A1.
To access the code, go to Tools > Macro > Visual Basic Editor
The simplified process of creating functions is fully described in this webpage:
Hope this helps. Let me know if you have any questions.