You can simulate multiple exposures in post-processing.
The custom setting you found is not for taking brackets of shots, it's simply for setting whether you want 1/2 stops or 1/3 stops as the units for partial-stop settings on the camera.
And yes, the 600D can perform autobracketing. You cannot, however, change the number of shots you can get in a set (3), and you can only adjust the range of the exposure covered up to 3EV in both directions (i.e., a 6EV range) with AEB. If you need to cover a larger range, you'll have to do it manually. The 600D also cannot do the HDR processing in-camera. You will have to do the HDR merging and tonemapping in post on the computer.
To use bracketing on the 600D, you have to set it in the camera menus (see pages 105-106 of the manual), and then take all the frames (i.e., press the shutter button three times, or use burst mode to take the three frames). You will not get the full bracketed set with a single shutter button push, unless you're using the Magic Lantern firmware add-on and its HDR Bracketing feature, also allows for more than 3 frames to be shot in a bracketed set.