Hi, sorry total noob here. I have crafted a very modest wild card (Mage - mirror entity) but I cannot find a way to get it into my deck because it is only viewable in the crafting screen. I assume when I play ranked I already play in “wild format” because several of my opponents have used this card against me! Also, when I look at the wild format cards in crafting, I do already have quite a few of those cards in my decks from packs etc. I must be fundamentally misunderstanding something, any help would be greatly appreciated 
sice you are new. Mirror entity is already in wild, but it also used in the core set as well.
So I managed to get around this only by converting my deck to wild format. It turns out I was playing in standard before, which raises one burning question - why are my opponents allowed to use the mirror entity card in standard when I am prevented from playing standard if it is in my deck ???
Every deck has to be set to a format. You do this when creating decks. Standard decks allow only standard cards. Wild decks allow all cards (with I believe 1 or 2 exceptions).
When starting a game, you select to play either on Standard ladder, or on Wild ladder (or casual). When you play Standard ladder, only Standard decks are allowed. When you play Wild ladder, all decks are allowed, both Standard decks and Wild decks.
So my guess is that so far you have been creating Standard decks and then playing with them on Wild ladder.