The function keys are typically dedicated to various features of OS-X on the Mac. In particular, F10, F11 and F12 are used to see the desktop in various views. This co-opts the F11 and F12 key which may be used as "Quick Keys" in HamSphere to enter /USERS, /USERS BAND, or /USERS QRG into the chat box using F11 and F12. (The latter two toggle with the F12 key.)
You may gain get the same functions in HamSphere on the Mac in at least two ways, one which respects the above assignment, the other which redefines some keystrokes to get those HamSphere functions.
Look at System Preferences under Keyboard. Whether the first line, "Use all F1, F2, ... etc. keys as standard function keys", is checked will determine how to do this.
If you check this box, HamSphere will respond to F11 and F12 as described in the HamSphere instructions. However, you will lose the dedicated functions which control volume, screen brightness, etc.
If you leave this box unchecked, you retain those dedicated functions of the function keys. However, you may go to Settings, the tab at the bottom of the HamSphere radio face illustration, cursor down, and enter the three commands in whatever boxes you see in the section which defines Ctrl-Function keys. Then click on Apply just before closing the window, and you should be able to get the HamSphere functionality by clicking the 'control' key before hitting the appropriate function key that you defined.
You need to make sure the blinking cursor is in the chat entry box over at the left before using the newly defined keys.
In my case, I entered /LOOKUP in the Ctrl F9 box, and /USERS, /USERS BAND, and /USERS QRG in the next three boxes. Now, I can position the cursor, press F12 to see under all the clutter above the desktop and press control-F12 to see who is on the frequency I'm on.
If you want to retain the dedicated functions and also avoid typing /USERS so much AND if you have a keyboard which has the FN key at the lower left, as does the Mac Bluetooth keyboard, for example, you may leave the box checked, and enter the commands under settings as described. Then, in order to get the control-Function sequence to work, you must first press the FN key, then the control key and the appropriate F key.
I wrote a much longer explanation which I am glad to send to anyone who requests it. I may post it on my HamSphere blog at another time.
PS Similar arrangements may be made for Windows, but they may depend on the platform.