First and most obvious question: are you sure you have power to the keyboard? Fresh batteries, or full charge if it's a rechargeable one?
Next, have you looked at the documentation that came with the keyboard to see if there's a way to initiate pairing? Some keyboards have a little button you have to press or hold to put it in pairing mode the first time it connects to your computer.
If that doesn't help, you may need to download and install the driver software for that keyboard. This shouldn't be necessary for the computer to at least recognize it, but it might.
The Logitech mk850 is definitely sold as Mac-compatible, so you should be able to get it working. Might need to borrow a USB keyboard/mouse to control the computer while you figure it out.
Hope this helps!
Theoden