The standard mac shortcut (Command+?) for opening the menu item search field doesn't work.
As Beibei mentioned - This is about cueing the cursor to the "Search" box of the help menu, not invoking the Adobe help system.
I've been annoyed by the same problem.
I posted this thread on MacRumors in the hopes of finding a solution. Will crosspost here if anything turns up.
Also note that the solution suggested by Scott above doesn't work because Illustrator doesn't seem to play nice with OSX system shortcut overrides. Also, this is about cueing the cursor to the "Search" box of the help menu, not invoking the Adobe help system.
the F1 key will bring up help.
You can alter the shortcut if you'd like by choosing Edit > Keyboard Shortcuts > Menu Commands > Help
There are a few areas in Adobe applications which do not honor the Mac OS shortcuts. This is generally because Apple implemented their shortcut years after Adobe implemented theirs. Therefore the Adobe shortcuts are generally favored since users are accustomed to them within the applications. And, since you can always change shortcuts, as I've outlined here, there's no need for Adobe to strictly stick to the Apple OS shortcuts in every instance.