Just bought a new electric cessna fron Hobby Lobby.Cessna 182 41.75 wing span.Just a cheap foamie but while starting to put it together noticed theres not dihedral brace,should I intall one?

do the instructions call for one? if not id say no

I looked at the plane on the HL site and couldn't tell if it has a full thickness airfoil wing or just the curved 1/8" foam airfoil. If there's no spar slot then it's so lite it wasn't included. I would still run two 18" long strips of 3/4" nylon strapping tape on the underside of the wing.
When a group of friends got into Lite-Stick pylon races, we used the tape to keep the wings from bowing up too much. The flight video didn't display too much bowing though; maybe the wing struts are req'd to help this. It looked like the pilot was having a bit of problems in the gusty wind.

The wing has some airfoil to it,not thin like the slow stick.Where they go together there's a small gap that a brace could fit into,but how would I secure? Epoxy on foam? Or special foam type glue..........

Foam-safe Ca, Epoxy, & Gorilla glue can all be used on foam models. Keep in mind though that Epoxy is heavy so if using it on a light foam model go lightly with it.