Since you mention using the Net-AIC we assume you are using it to convert from the RS232 output of the Micrologix to RS485 from its port 3.
1. Using the Net-AIC manual hook the 1500 to it using the proper cable. You can use power through the PLC cable or supply 24VDC seperately. Set the power switch accordingly.
2. Wire from port 3 to your device. I have no idea what your 'tank sensor' connections look like.
3. In the PLC program, while offline, set the PLC port to Modbus RTU Master. Set the other parameters (baud rate etc.) as needed by the end device.
4. In the program create a MSG to read a known value from the device. Look in the device documentation for the proper Modbus address. (The address you end up using may possibly have to be 1 higher.) Each Message will have to use an MG:xx file
5. Select the channel, the Modbus command (for more on Modbus itself go to
www.modbus.org), the data table within the PLC where the information will end up and the size in elements (probably 1 for the time being)
6. Enter the Modbus address of the device and its node address.
7. The commend is edge triggered. Don't try to re-trigger it until it DN or ER bits turn on.
8. A good troubleshooting method if all else faile is to swap the two RS485 data lines.