Hi
try
Me.txtTotalPaid = Nz(DLookup("Total", "qryMembSumPayments", "mpClientID=" & me!mpClientID), 0)
If mpClientID originates in the main form, use the parent.
Me.txtTotalPaid = Nz(DLookup("Total", "qryMembSumPayments", "mpClientID=" & parent!mpClientID), 0)