How do I get the key from lights out as I have tried for ages and can't..

It is hidden and hard to find.

Look for a platform that raises you up.

It is on a platform that raises you up, it is hard to find though.

I assume you mean the key needed to get into the second branch off Lights Out. The key is NOT in Lights Out - it is in the Temple of Darkness, a bit further on. Collect the three cortex tokens; One is at the bottom of a stack of boxes with a TNT on the top, one is just past a snake (which you must jump on, NOT spin) in a lump along with a TNT (that the snake would hit), and one is on an invisible branch-off near the end. Run off the edge to the left when there is a Wumpa Fruit there. Then do the cortex level.

First, you have to get the Blue Gem in Toxic Waste. From there you just try to get all the boxes in Jaws Of Darkness. Once you see the blue gem go on them and it shall take you from there. Then get the Cortex Heads to get the key. Now you have the secret level Fumbling In The Dark.Also if your wondering to get the other key, just go to Sunset Vista and try to get all the boxes and the Cortex Heads once you completed the bonus round you will have a key and you will be able to use it for the other secret level Whole Hog.

You Gets Fall, The Key Not In Light out, The Key In Jaws Of Darkness , Get The Box Have Cortex Face O_O.
Complete The Bonus And Get The Key From cortex (NOTE : Don't Die In THE Bonus Or You Have Retry The Level Again) (NOTE 2 : Don't Exit The Level After You Get The Key Or You Have Retry the Level)Complete The Level And You Well Back To The Map And Return To See Your secret Level.

Man , The Key Not from The Light Out , it's On Jaws Of Darkness , Try Find Cortex Bonus Round And COMPLETE HIM WITHOUT DIES , So You ll Get The Key From Cortex