I purchased the "Echo Knight" Martial archetype from the Wildemount source book (as a standalone purchase) - I have paid for it and the transaction has been processed but when I am editing my Fighter, it does not appear in the drop-down list of Martial archetypes?

The character was initialised before this purchase was completed, do I need to rebuild from scratch to get the Echo Knight option? 

Is Critical Role Content enabled from the Home section of the Character Builder? If not - you won't see it.

Thanks Emmber, my mistake for not realising that EGtW was a Critical Role thing (I am yet to delve into their content and only bought Echo Knight as I found it through googling Fighter builds!)