From within the EndNote Desktop program, click on Library and then Sync.
Follow the instructions to sign up for an account for the online version. If you already have an EndNote Online account, enter your e-mail address and password for your EndNote Online account and press OK.
Setting Sync preferences
Sync settings are editable within EndNote Desktop:
Go to the Edit menu, select Preferences... and then click on Sync from the left side of the window that appears.
You can change your email address or password and also choose whether to sync automatically.