Hi, welcome to justanswer.com! My name is Mike.I am going over your question and I will be with you shortly.
First Download the Logitech Options and install it: http://support.logitech.com/en_us/product/multi-device-keyboard-k480
Most Logitech products are equipped with a Connect button and will have a Bluetooth Status LED. Usually the pairing sequence is started by holding down the Connect button until the LED starts blinking rapidly. This indicates that the device is ready for pairing.
Then open System Preferences and click Bluetooth.Select the Logitech device you want to connect to from the Devices list and click Pair.Follow the onscreen instructions to finish pairing.Upon pairing, the LED light on your Logitech device stops blinking and glows steady for 5 seconds. The light then turns off to save energy.
There are very clear directions for that unit here: http://www.logitech.com/en-us/manuals/k480-immersion-guide
Select First Time Setup
To connect to an Apple Macintosh, iPhone®, or iPad®:Press and hold down the “i” connect button for 3 seconds.
Were you able to connect the keyboard?