Well the main problem was that I hadn't included the ContactID field in the main query! Once I did that I was able to figure out the correct syntax to reference the textbox holding the ID. Here's the final code in case it helps someone else.
Code:
Dim db As Database
Dim rs As Recordset
Dim rsFiltered As Recordset
Dim strSQL As String
Dim strAddr As String
'This one assembles the Addr1 and Addr2 in the query
'strSQL = "SELECT DISTINCTROW [tblClients/Prospects].CompName, tblCompanyContacts.ContactName, tblCompanyContacts.ContactID, tblContactAddresses.Attn, IIf(IsNull([Addr2]),[Addr1],[Addr1] & ', ' & [Addr2]) AS Street, tblContactAddresses.Zip, tblContactAddresses.City, tblContactAddresses.State, tblContactAddresses.Default " & _
"FROM ([tblClients/Prospects] INNER JOIN tblCompanyContacts ON [tblClients/Prospects].CompID = tblCompanyContacts.CompID) INNER JOIN tblContactAddresses ON tblCompanyContacts.ContactID = tblContactAddresses.ContactID " & _
"ORDER BY [tblClients/Prospects].CompName, tblCompanyContacts.ContactName;"
' This one assembles the Addr1 and Addr2 later on
strSQL = "SELECT DISTINCTROW [tblClients/Prospects].CompName, tblCompanyContacts.ContactName, tblCompanyContacts.ContactID, tblContactAddresses.Attn, tblContactAddresses.Addr1, tblContactAddresses.Addr2, tblContactAddresses.Zip, tblContactAddresses.City, tblContactAddresses.State, tblContactAddresses.Default " & _
"FROM ([tblClients/Prospects] INNER JOIN tblCompanyContacts ON [tblClients/Prospects].CompID = tblCompanyContacts.CompID) INNER JOIN tblContactAddresses ON tblCompanyContacts.ContactID = tblContactAddresses.ContactID " & _
"ORDER BY [tblClients/Prospects].CompName, tblCompanyContacts.ContactName;"
Set db = CurrentDb
Set rs = db.OpenRecordset(strSQL, dbOpenDynaset)
rs.MoveLast
rs.Filter = "ContactID = " & [Forms]![frmCompanyContacts]![txtContactID] & " AND Default=True"
Set rsFiltered = rs.OpenRecordset
rsFiltered.MoveLast
If rsFiltered("Addr2") > "" Then
strAddr = rsFiltered("Addr1") & ", " & rsFiltered("Addr2") & ", " & rsFiltered("City") & ", " & rsFiltered("State") & ", " & rsFiltered("Zip")
Else
strAddr = rsFiltered("Addr1") & ", " & rsFiltered("City") & ", " & rsFiltered("State") & ", " & rsFiltered("Zip")
End If
Me.txtAddressInfo = strAddr
rsFiltered.MoveNext
rs.Close
rsFiltered.Close
db.Close
Set rs = Nothing: Set rsFiltered = Nothing: Set db = Nothing