The Express Pass benefit is included with your Universal hotel reservation (not the preferred hotels, just Hard Rock, Royal Pacific and Portofino Bay offer this benefit). It includes unlimited front of the line access for all attractions that have it (Forbidden Journey, for example, doesn't allow have Express pass access but most rides do) There is no additional fee, all you do is show your room key to get access.
There is also an Express pass that you can purchase, that you mentioned, that is rather expensive and unless you buy the really expensive version it only allows you to the front of the line once.
We have never purchased the dining plan, I understand it includes a limited number of counter service restaurants only and it's not that great of a deal.