I defined a few functions in a workbook using VBA, and then expected to be able to use them in a cell formula - but Excel does not recognise the function. I just get #NAME?<
I faced the same issue, after struggling around following worked for me:
My function was inside a module in macro workbook called Personal.XLSB. I prefixed the function name with the personal macro workbook filename and !, so if function name is theFunction(x,y) I entered in cell "=PERSONAL.XLSB!theFunction(x,y). This worked.
Please note that PERSONAL.XLSB is always open in hidden mode for me.