You must use the "multi-hop" message type on the PLC-5 to communicate with a 1761-NET-ENI device.
An excellent example of this kind of MSG instruction many be found in RA Knowledgebase
Document # 19726.
Your PLC5E must have a minimum firmware level to be able to communicate with the Net-ENI or with ControlLogix:
Series E rev D.1
Series D rev E.1
Series C rev N.1