This is how you can pick the row and column from the caption of the button, and from the selection in combobox, and then reference that cell.

Code:
Private Sub CommandButton1_Click()
     Dim lngRow as Long, lngCol as Long
     lngRow = buildgPlan.CommandButton1.Caption'Get Row
     lngCol = Me.ComboBoxDeterminingColumn.Value
    .Cells(lngRow, lngCol).Value = Me.ComboBox1.Value    
    End Sub