Hello Sandra and welcome
Sorry you haven't received a reply to this question before now, but most of us are over on the UK DIS, which you can join by clicking on the link in my signature, anyway, to answer your question - the plusses are in addition to the park days e.g. a 5 day park hopper plus, gives you 5 days for the 4 main WDW parks, PLUS two days to either spend at the water parks, or on Pleasure Island etc.
The company you mention is not one I've heard of Brits using, but
www.ticketmania.com is often used and recommended if you want to book on-line, also check out
TicketRes or you could book with Keith Prowse, or check your local
Disney Store, however, they do not have all types of passes available.
If you'd like to join us on the UK DIS, you will need to register to post