I assume that you have put a breakpoint on the
MSComm.Output = "A" + CHR$(13)
statement and that you do get to it.

I also assume that you have put a scope on the signal line and seen no signal. (just ensuring that the problem is within the PC and not because the receiver is not enabled on the 8051)

If this is the case, I think you should investigate the OnComm event which determines when the serial buffer is considered full and starts transmitting. I think the MSComm property you should be looking at is SThreshold.

Another property is the Handshaking property which determines whether you use the CTS/RTS signals (set to comNone for no handshaking).

Perhaps you could try sending data without receiving anything just to get the port to operate. I have taken this extract from my book so it should work- try it (obviously you will need to modify the UserForm object to your setup)

Sub Initiate()
Dim TransmitBuffer As String
With UserForm1.MSComm1
.CommPort = 1
.Handshaking = comNone
'no handshaking required
.Settings = "9600,N,8,2"
'set for 9600 baud, no parity, 8 bits, 2 stop bit
.DTREnable = True
.RTSEnable = True
End With
UserForm1.MSComm1.PortOpen = True
UserForm1.MSComm1.InputMode = comInputModeText
'text data
TransmitBuffer = "A" + Chr(13)
UserForm1.MSComm1.Output = TransmitBuffer
End Sub

Good Luck!
Aubrey