Comment 2 for bug 1419345

For the benefit of anyone searching for this and as ~suv suggests:

It looks like the way of adding symbols has been changed since the release notes were written.
The symbols dialog can be opened by going to Object->Symbols (Shift-Ctrl-Y)

A symbol can be added by selecting a group of objects and clicking on the "Add symbol from the current document" button at the bottom left of the Symbols sidebar (see attachment)