The chorus FX unit can be found in the FX section of ATOM:

The unit begins as the fourth FX unit from the top, and can be turned on by clicking the top right power icon or the left-side light in the Chorus module of the fx rack section:

"Sync," syncs the Chorus effect to the BPM of your DAW.

"Rate," determines how quickly the modulation effect occurs. Higher rate values mean faster chorus effect changes.

Delay provides a more fine-tuned way to modulate the rate. 100% Delay will make the effect identical to the rate, but lower values will make the effect slightly slower than the rate shows.

Depth refers to how intensely the chorus effect is applied, while FDBCK (or feedback) determines how long or short the chorus effect on the sound is. Higher FDBCK values will have longer "tails," or essentially more decay.

"Mix," is essentially a dry/wet knob for the Chorus. A Mix of 0 means no chorus effect is applied, while a Mix of 100 means the entire signal is run through the chorus modulation.