While you should probably put your function in a regular code module,
you CAN call it like this (assuming it's in the Sheet1 code module):

=Sheet1.Cuberoot(x)

Note that I'm assuming you meant

Public Function CubeRoot(num As Double) As Double
CubeRoot = num ^ (1 / 3)
End Function

In article ,
JCDW wrote:

In both Excel 2000 and 2007 when I enter the following functions on a
worksheet module (or the Excel obect page 'behind' the worksheet), I am
unable to execute (or call) on the worksheet. I get the error, [ #NAME? ].

Sub ShowSum()
Sum = 1 + 1
MsgBox "The answer is " & Sum
End Sub

Function CubeRoot() ' returns the cube root of a number
CubeRoot = num ^ (1 / 3)
End Function