Hi LK,
To use the function in a worksheet, you have to either have that function code in a module of the worksheet you want to use it on, or in a module of an addin that is installed/active in excel.
For example, though I know you know how to make functions, create a new workbook in excel and press alt-f11 to open the VBA editor. Right click within the vbaproject of that new workbook, go to Insert, then Module. In that module, paste the following:
Function DoubleANumber(ByVal TheNumber As Double) As Double
DoubleANumber = 2 * TheNumber
End Function
Close the VBA editor, and in a cell of that worksheet, enter =doubleanumber(5) and you'll see 10.
As I said, if you have a custom add-in, or one that allows you to make changes, if you put that function in there too it will work, provided the add-in is installed.
Matt