Where is the add a page plus sign in pages 5?

In every version of Pages I have used (back to v1.0 in iWork ’05) as well as AppleWorks before it, the “return” key & the “enter” key have different functions. A page break can be added by typing ⌘enter. You must use the enter key on the number pad, not the return key above the right side shift key. If you don’t have a keyboard with a number pad, you should user ⌘+fn+return.

I tested all of this just now in Pages 5.0.1 in El Capitan, Pages 7.3 in High Sierra, Pages 4.3 in High Sierra and AppleWorks 6.2.9 in Snow Leopard.

Just type command Enter in your text, or:

Menu > Insert > Page Break

The default toolbar in Pages v7.3 has it:

and if somehow, you removed it, you can add it back again from the View menu : Customize Toolbar… panel by dragging and dropping the individual icon back onto the main Pages Toolbar.

There are other means to add a page, either via the adjacent Insert button, or from the Insert menu — as Page Break.

I just ran El Capitan 10.11.6 in VirtualBox v6.n and Pages v5.6.2 has no + Add Page toolbar item either.

I just checked Pages 5.0.1 under El Capitan & there is no "Add Page" option from the toolbar. The Insert button doesn't have it either. The closest you can get from either the Insert button or a menu item is to add a page break.

As Peggy has pointed out Return is not Enter.

What about the Menu option?

You can assign your own keyboard shortcut to Page Break in System Preferences > Keyboard if you like.