promo cards are usually distributed by arcane wonders at exhibitions such as origns and gen con conventions. they sometimes send them to tournament organizers as prizes for folks.
as far as when they are allowed, usually not allowed in major real life events. as it advantages those who have been able to obtain copies over those who have not. however, in many online octgn tournaments they are allowed - see each event for exclusions/modifications.
casual online matches are usually a free for all approach.
you should use the non-promo released version of the octgn cards if they have been officially released in a set as they will have up to date verbiage on them.
as a side note:
you probably should check with your opponent before you load decks for rules they may be expecting to operate under. there are differences in various players expectations on how certain cards function - such as killer bees.
good luck. hope to see you at live events or online soon.
