I upgraded to Photoshop CC 2015 and noticed that some of essential plugins (ie. LayerCraft) don't appear in CC 2015. Should I reinstall CC 2014 or any other solutions to make the LayerCraft appear under Extensions?
Yes, you have to reinstall your third party plugins for CC 2015. Copying them is likely to break some of them (because their support files won't be moved with them). Also, check with the plugin maker for updates, because several plugins needed bugs to be fixed before they worked correctly in CC 2015.