Concatenating the value list into a string expression will work if you build
the SQL statement in code and then set the RecordSource of a form or report
to the SQL statement or open a recordset based on it.  It won't work in a
saved query, however; that requires literal values in the IN operator's value
list.  You have to use one of the methods described in the MSKB article to
which I gave you the link.  I usually use the InParam and GetToken functions
for this.  Its quite simple to implement; you just put the following
functions in a standard module in the database (these might differ slightly
from those currently in the article as they are ones I've been using for
years)

'''''''''Module Begins'''''''''''
Option Compare Database
Option Explicit
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Function GetToken(stLn, stDelim)

   Dim iDelim As Integer, stToken As String
   iDelim = InStr(1, stLn, stDelim)
   If (iDelim <> 0) Then
       stToken = LTrim$(RTrim$(Mid$(stLn, 1, iDelim - 1)))
       stLn = Mid$(stLn, iDelim + 1)
   Else
       stToken = LTrim$(RTrim$(Mid$(stLn, 1)))
       stLn = ""
   End If
   GetToken = stToken

End Function
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Function InParam(fld, Param) As Boolean

   Dim stToken As String
   If IsNull(fld) Then fld = ""
   Do While (Len(Param) > 0)
       stToken = GetToken(Param, ",")
       If stToken = LTrim$(RTrim$(fld)) Then
           InParam = TRUE
           Exit Function
       Else
           InParam = FALSE
       End If
   Loop
   
End Function
'''''''''Module Ends'''''''''''

You then call the InParam function in the query by passing the value of the
field and a reference to the parameter into it.  In your case the latter is
the txtNetworks control on your form, so you'd call the function in the query
along these lines:

SELECT *
FROM YourTable
WHERE InParam(Networks,Forms!frmWeeklyBroadcastSchedule!txtNetworks);

One thing to note here is that the comma delimited value list in the control
does not need text values to be delimited with quotes as would be the case if
you were using the IN operator itself with a list of text values.

Ken Sheridan
Stafford, England