Connect the audio interface via USB or FireWire. Be sure the software and firmware are properly installed by checking the Preferences tab in the Tool menu. It should show a computer sound card and the interface as option; select the interface as source. Check the manufacturer's website for all updates and install if applicable. Restart computer.
Connect the unit at the back of the chain (post effect). The rear of the interface will have either RCA, digital or ¼ outputs. You will need appropriate cords and/or adapters. Effects units have ¼ inputs. Use the appropriate cord to plug the output of interface to the input of the effects unit. Outputs of the effects unit should go to your master board inputs or sound source input. This configuration will affect the total mix post computer, meaning it will send a blanket signal to the entire mix without the ability to edit. Use this as a mastering effects unit to a post master console.
Connect the unit at the front of the chain (pre-effect). Connect from the outputs of the effects unit (rear) to the front of the interface, typically ¼ connections. The gain on the interface will now control volume and secondary effect gain. The microphone or instrument will be connected directly into the effect input or via a preamp. Keep in mind, the effect will be recorded onto the track without the ability to remove.