msgbox apear if macro clicked

hi
i was wondering if you tell me the code so that if i click a macro it would prompt a msg box if the person has chosen a cell with a value on it

for example

atm i have a macro that lets me copy date from one cell to another

and with that macro, the cell that is chosen is where the macro copies on, but i want so that if the user chooses a cell with a value then it would prompt a msg box saying "choose a empty cell"


if you could help me then that would be great

thanks in advance

Private Sub CommandButton1_Click()
ActiveCell.Value = ActiveCell.Value + Sheet11.Cells.Range("D12").Value
End Sub

sorry about that

thanks it works