Two type of corner stretchers: nailers and sleepers. For nailers, just select a back and then add stretcher. You can add the nailer on the inside of the back or the outside. for a sleeper, choose toe kick and a back then add stretcher.
One problem. I add the stretcher and then try to align it with the top of the cabinet. I type in the number 0 for Inset 1 and it keeps changed to -0.1875. No matter what I try, it won't let me set Inset 1 to 0.
Don posted the cabinet to the beta site and I have tracked the issue down. Seems that the corner cabinet "back" style stretchers will position off by half of the full dado depth (and have ever since their introduction). I have this listed and should be resolved for the next release. You can get around this for now by typing in half of the dado depth for inset #1 and press enter. Doing this will get it to positioned at zero.