The Chakram launcher should be available to you immediately. Once you find the first weapon bench you should be able to select it under the list of available assault rifles. Alternately, if you find a weapon during a mission, you can choose to change your loadout and equip the weapon that way.
If the weapon isn't showing up, it might be that it wasn't unlocked. In order to unlock the Chakram Launcher, you have to get to the end of the 45 minute timed segment of the Kingdomes of Amalur demo.
The Reckoner Knight armor should be available once you return to the Normandy after the Citadel section. If you go to the armory, there is an area that allows you to change your armor. The second slider down allows you to select the Reckoner armor.
Regarding your EA accounts, no additional linking should be necessary as long as you're playing with the same account/console that you played the demo on.