how do I access the "Flames of the Inquisition" weapons and armor ?

For weapons, you must own Flames of the Inquisition Weapons Arsenal DLC, which was available with pre-orders and Deluxe Edition of Dragon Age: Inquisition.

Once you have this DLC, there are few tiers of weapons and armor. You get weapons from the first tier in Haven, in a crate next to place where you wake up. You will also get schematics for weapons, available once you reach Skyhold.

For armor, you need either Dragon Age: Inquisition Deluxe Edition or Dragon Age: Inquisition Inquisitor's Edition. The armor can be acquired in Haven. It is located in a chest named "Special Deliveries", the same one which contains the Flames of the Inquisition Weapons Arsenal. There is a version of armor specific for each class;

Heavy Armor of the Dragon (Warrior)
Medium Armor of the Dragon (Rogue)
Light Armor of the Dragon (Mage)
Once you reach Skyhold, you will find crafting schematics for all 3 armors in a chest located in the Undercroft, along with the weapon schematics for the Flames of the Inquisition Arsenal.