There are a whole slew of things, to try, and you might have done them already. First, which operating system are you running?
Now on for the tests:
1. Do both left and right USB ports work? Try other devices like flash thumb drives, do both left and right work, by this can you read and write to them when they are connected directly to the ports, no hub.
2. Have you tried both ports directly with the keyboard? Again, if you are using a hub, take it out and connect directly.
3. Did this work before on an older OS X? What about on a Windows PC, does it work on other computers?
4. Load the App Store and see if there is a new driver for the keyboard.
5. If App Store doesn't have a driver for the keyboard, have you tried directly with Microsoft to get the Mac driver (this is not always possible, but you might want to try to see if Microsoft has a OS X driver.
6. If you must use a hub, please test without it first, but if that is the source of the problem, then you might need to get a powered hub. I never use other than powered hubs as I don't want to tax my ports that much.
7. I suppose someone should ask, but why do you need an additional keyboard (Microsoft or otherwise)? "I just prefer it" is an acceptable answer. My personal preference is for the Mac built in keyboard or one to the other wired keyboard or Bluetooth keyboard. I also have a Logitech wireless and another wired one and they both work. If you are using Fusion/Parallels/Boot Camp to run Windows on your Mac, please note that command works as the Windows key.
8. With keyboard plugged in reboot.
9. Reboot, login and wait a bit, then plug in the keyboard. This will cause an interrupt and may cause it to download the driver.
FYI, I just checked Microsoft and they do have a driver for OS X at: http://www.microsoft.com/hardware/en-us/d/natural-ergonomic-keyboard-4000
but it seems to have stopped at Lion (10.7) but you might want to try it anyway. But this is why we also need to know which OS X version you are running. If you are still on Lion, then you should be in luck with the Microsoft driver for OS X.