Confused.com... Sumproduct or Sumif???

Ok, thank you so far, I can see it worked for the other guy... as I say, (Sorry!) I am a novice. What do I do with this?

I've added the developer tab to the ribbon. And I've got to the point where I can create a macro. What do I do from this point?

And, if I wanted to apply this rule to D15 (add a number) to B15 (Sum) would the macro read as follows?

Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Columns(2)) Is Nothing Then Exit Sub

Application.EnableEvents = False
Range("B15"&Target.Row) = Range("B15"&Target.Row) + Range("D15"&Target.Row)
Range("D15"&Target.Row) = ""
Application.EnableEvents = True

End Sub

Would I follow this process?
(Select the cell to which I am to apply this to?)
Macro
Macro name "Add"
Create
Copy and paste the above macro to that specifc cell?

I have tried this, it doesn't seem to work. like I say... true novice

Where am I going wrong? I would like to apply this rule to the columns above an below the rows cited

Many thanks...