If any of you are like me, you have probably wracked your brain trying to figure this out, and may have even thought this enchantment was bugged.
In order to add Shadowflame Wreathe to your weapon/s, you need to go to a Fyrakk Assault and do this at a “Shadowflame Wreathe Incantation Table”. Blizzard will not allow me to add images or link to this post, but you can easily do an internet search for this. The table looks like a regular, plain stone table with 2 red sheets of cloth on it, and 2 red orbs.
Blizzard, please consider adding this information to the tooltip of the enchantment to help out the countless gamers who have add this very same issue. It will save a lot of players from the stress of not being able to use this enchantment.