Ensure the computer is powered off. Connect the audio interface to the computer using the relevant data cable. Connect a pair of stereo inputs from the interface to the input plugs on the compressor. Connect the compressor outputs to an output on the audio interface. Make a note of which inputs and outputs were used. Assuming the interface has more than one set, each pair of connectors will be numbered.
Power on the computer, then launch Logic Pro by clicking on the "Platinum Album" icon. Use the dialog box that appears during the startup procedure to navigate to the Logic project on which you want to add the compressor. Double-click the project to open it.
Click the "Mixer" tab at the bottom of the screen to open the Mixer view. Click the drop-down "Options" button at the top of the Mixer window and select "Create new auxiliary channel strips." Scroll to the far right of the mixer and locate the new Auxiliary channel. It will appear between the channel strips and the final Mastering channel.
Click the gray buttons above the volume fader on the auxiliary channel strip. The upper button selects the input, and the lower one controls the output. For the input, navigate to "Input" and select the numbered input to which you connected the compressor. Select the connected output from the list on the lower gray button. Click one of the dark gray buttons above the input and output area on the channel strip. Navigate to "Bus" and click on an available numbered Bus.
Select the audio channel to be routed to the compressor. Click on one of the dark gray buttons in the Bus section of the channel fader. Select the same numbered Bus from the drop-down menu. Turn up the rotary dial next to the button until it reads "0.0" in the adjacent display. Play the track and adjust the dial until the level on the auxiliary track is as high as possible without tipping over into the red.
Create a new channel in the "arrange" window by clicking the "+" button at the top. In the channel strip for the new channel, click the gray input button and select the numbered bus as the input. This new channel will contain the processed audio.