I tried the Ctrl + Shift + + and also didn't work. Where did you see it?
Anyway, here is a workaround.
Enter the text including the sub- or super-script.
Simplify the text layer.
Use the Rectangular Marquee to select the script.
Activate the Free Transform tool (Ctrl-T). Use the arrow keys to position the script, and optionally resize it as in this example.
Downside is that you cannot edit the text since it has been simplified.
