Calculating Average ...of a dynamic data.

Hmm... I confess I have zero experience using DDE's but you would need to adopt a different event as already outlined given DDE udpates do not invoke a Change event, rather you need to use the Calculate event, however, the problem here is that technically you're open to error if the sheet recalculates on the basis of some action other than DDE update having taken place... if this is a reality we should discuss further, if not, that is to say only DDE's will be recalculating then perhaps try:

Private Sub Worksheet_Calculate()
Range("B1").Value = Range("B1").Value + Val(Target)
Range("C1").Value = Range("C1").Value + 1
End Sub

That said, given the above to all intents & purposes becomes a Volatile Event you could just as easily dispense with VBA, enable Iteration with Max Iteration of 1 (to permit circular references) and enter formulae instead:

B1: =B1+A1
C1: =C1+1

Just tried your latest VBA code , its now working with the DDE update , the only problem now is B1 is showing zero (basically not summing up A1 ) but C1 is counting fine .

That's because I'm a complete doofus..., should be:

Private Sub Worksheet_Calculate()
Range("B1").Value = Val(Range("B1").Value) + Val(Range("A1").Value)
Range("C1").Value = Range("C1").Value + 1
End Sub

Re: learning VBA - self taught I'm afraid... by self I mean: Macro Recorder, Forums like this one... there are a good few books about - Walkenbach always get good reviews, PED for the more advanced programmer...