If your tracks are in 24 bit there are 2 way to dither -
either track by track or in the Master Stereo fader of the song

Either way, you must
1) open your Mixer Window
2) click and hold on your INSERTS > Stereo > Logic > Helper > Dither
3) choose your bit depth
4) Go to the Master Fader and hit the BOUNCE key
5) set your bounce parameters
6) hit Bounce again
Once the bounce is done you will have your Dithered track or tracks...