Screenshot

This question was asked and answered in 2009, but I don't know how to fix it. When I try to paste screenshots using command-control-shift-4, it will create a box to take the screenshot, but when I try to paste it, it will not paste. This is only happening in keynote 8.0 (I've tested it in pages and mail and it worked as usual), and it has never happened before. The answer that seemed to fix it in 2009 was: ~/Library/Preferences/com.apple.screencapture.plist. Where do I put this command?