iCloud in doc
How do I add an iCloud icon in a 2008 Mac's dock? El Capitan v 10.11.6
iMac
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If you mean iCloud Drive, one method that works with Mojave and I suspect will also work with El Capitan:
Go to Finder View menu and select "Show Path Bar" if that isn't already on.
Go to Finder "Go" menu and select iCloud Drive.
From the Finder window Path Bar (bottom of the window), drag iCloud Drive to the right hand segment of the task bar.