You are going to end up with way too many buttons.
A DoubleClick event might serve you better.
If you put this in the code module for Sheet1, double clicking on any of the filled cells in column C will add the value in D to the others on the right.
Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
Dim MatchingCell As Range
With Target
If .Column = 3 And .Value <> vbNullString Then
Cancel = True
Set MatchingCell = Me.Range("H:H").Find(what:=.Value, LookIn:=xlValues, MatchCase:=False)
If Not MatchingCell Is Nothing Then
With MatchingCell.Offset(0, 1)
.Value = Val(CStr(.Value) + Val(CStr(Target.Offset(0, 1).Value)))
Beep
End With
End If
End If
End With
End Sub