Select the "Live Devices" menu on the left of Ableton.
Select the "Audio Effects" folder.
Drag and drop the "Looper" effect to the Audio or MIDI channel you plan to loop. If you want to loop audio from the entire track, drop "Looper" into the Master channel.
Click the "MIDI" box at the top right of Ableton. This will enable MIDI mapping, letting you customize parameter controls with your Akai APC40.
Click on the Device Activator (the circle with a line in the middle at the top left of the effect device) on "Looper."
Press one of the buttons on your Akai APC40. Doing so assigns the Device Activator parameter to the button you have selected.
Click on Record (the large box with a circle in the middle on the left of "Looper"). Press a button on your Akai APC40 to assign the Record parameter to the button.
Click on Play (the triangle in the audio viewer of "Looper"). Press a button on your Akai APC40 to assign the Play parameter to the button.
Click on Stop (the square in the audio viewer of "Looper"). Press a button on your Akai APC40 to assign the Stop parameter to the button.
Click the "MIDI" box at top right to close MIDI mapping. Your APC40 will now be controlling the parameters with the buttons you have selected.
Play the audio or MIDI file you plan to loop.
Use the Akai APC40 buttons that you have assigned to record a segment.
Hit the button assigned to play the clip, and the audio will loop.