You have to select a Dynamic UDP circuit type in the USS to connect via IP to a Bristol RTU.
When you configure the Dynamic UDP circuit, be sure to enter the IP address of the PC running the USS in the Local Host field and 1234 for the Port number.
When you configure the device in USS go to the Circuit Options tab and click on the Circuit Options button to enter the IP address for the Bristol RTU(end point) and Port number (1234).
You do not need the ACCOL file for the Universal IO Server configuration. All you need to know is the Bristol signal names. A good test to check the configuration would be to connect with an OPC client and request the time ( #TIME.007 ) from the RTU.