try this:

Dim dbs As Database
Dim qdf As QueryDef
Dim rst As Recordset
   Set dbs = CurrentDb
   Set qdf = dbs.CreateQueryDef("")
   qdf.sql = "SELECT TM_InvoiceItemGroup.Field3, TM_CompContact.AcNo
FROM TM_InvoiceItemGroup LEFT JOIN TM_CompContact ON
TM_InvoiceItemGroup.Field3 = TM_CompContact.AcNo
WHERE (((TM_CompContact.AcNo) Is Null))"
       Set rst = qdf.OpenRecordset(2, 512)
   rst.movefirst
   while not(rst.eof)
       xxxxxxxxx
       xxxxxxxxxxx
       rst.movenext
   wend

write your code to update instead of xxxxxxxxx

You can step through any recordset using code like:

Dim dbCurr As DAO.Database
Dim rsCurr As DAO.Recordset
Dim strSQL As String

 strSQL = "SELECT TM_InvoiceItemGroup.Field3, " & _
   "TM_CompContact.AcNo " & _
   "FROM TM_InvoiceItemGroup " & _
   "LEFT JOIN TM_CompContact " & _
   "ON TM_InvoiceItemGroup.Field3 = " & _
   "TM_CompContact.AcNo " & _
   "WHERE (((TM_CompContact.AcNo) Is Null))"

 Set dbCurr = CurrentDb()
 Set rsCurr = dbCurr.OpenRecordset(strSQL)
 Do Until rsCurr.EOF
   ' do work here
   rsCurr.MoveNext
 Loop

Having said that, though, it's seldom efficient to loop through a recordset
in VBA if you can simply use a SQL query, and from the sounds of it, you
should be able to create an INSERT INTO query that simply generates the
necessary records. Unfortunately, you haven't given enough details for me to
actually write the SQL for you.