For the event there’s a special arena and I have no idea how to add a wolf to the empty slot so i can play :(

Are ya still having problems, if so can you describe where and what's going wrong a little more? Is it the Murkwood Trials on the event shop page, or are you having problems adding wolves to your team in the campaign area? Or the event Arena game mode, where you combine the pre-selected roster of wolf breeds and companions into a team?

Sorry if this seems redundant, I'm just tryin' to figure out exactly where you're stuck so if you still need help it'll be easier to figure out whatcha need.

You can't add a wolf, instead you can buy 1 creature of your choice from the arena shop to add to your team! It works similarly to the normal arena I believe, so this advanced arena guide will probably be able to explain a lot of the mechanics :]

Here's a screenshot to illustrate if that helps. Ignore the Volmyr looking like one of my wolves haha, that's not literally one of my wolves but just uses a skin to have the same appearance (skins can be changed in the main non-event Arena page).

Buying creatures to add to your team from the shop casts Paws, the arena currency, you can see how many you have just above your synergies. You start with 1 Paws to buy a creature of your choice (all shop creatures cost on 1 Paws, as far as I can tell.)

The guide I linked has a lot of helpful information, but for a few basic tips: You unlock more active creature slots as you level up, you can see your progress to leveling up with the XP bar above Paws. You get a free refresh of the shop after every completed round, and can also refresh again as many times as you want for Paws after you've used your free refresh (costs 2 Paws per non-free refresh iirc, but don't quote me on that). Lastly, if you want to get 2 or 3 star creatures you have to merge them - every 3 copies of a creature with the same star level will automatically merge to become a higher level. For example, if you have 3 1-star copies of a creature you can merge those to get a 2-star copy. For a 3-star copy, you'll need to merge 3 2-star copies. 3 is the highest star level you can achieve, as far as I know.

Hopefully this helps a bit, and you enjoy the arena! :]