Hi,
Messages has two accounts already set up (to some extent) which cannot be deleted.
The iMessages one requires you add your Apple ID and Password.
You cannot delete this account - only remove the Apple ID and retype another.
The other is the Bonjour account which by Default is Off (the "Enable this Account" is not ticked).
The Add option in the Messages Menu is only for Jabber (Google) AIM and Yahoo accounts
The + icon button at the bottom of the Accounts list will also get you to the Add Accounts screens.
You cannot add/ Start the iMessages Account from System Preferences > iCloud as Messages and iMessages in not part of iCloud.

9:03 pm Thursday; August 11, 2016
iMac 2.5Ghz i5 2011 (El Capitan)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone and an iPad (2)