In the VBA editor (Alt-F11 to get to it) double click on the Sheet you want this to happen on.
Enter the following code:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
If Application.Intersect(Range("B:B"), Range(Target.Address)) Is Nothing Then
Exit Sub
End If
If Target.Address <> "$B$24" Then Exit Sub
If IsEmpty(Range("D24")) Then Exit Sub
If Not IsNumeric(Target.Value) Then Exit Sub
If Not IsNumeric(Range("D24").Value) Then Exit Sub
Range("D24").Value = Range("D24").Value + Target.Value
End Sub
Hopefully the comments are enough to guide you on what is going on. The Worksheet_Change event is fired whenever something changes on that particular sheet.