Currently, NSU is offering version EN20 for free licensed download. You must update it to the most current version on both PCs and Macs. It is not advised to install previous versions. You can also sign up for the free EndNote Online version, which can be synced with a desktop library or used on its own. (If you have a Chromebook -- the desktop software cannot be installed on Chromebooks -- but you can use the basic online version.)
Do not save the EndNote Library in the iCloud!