I just played my first game and I love it! I have one question though. How is the monster that joins determined? I tried to look in the pamphlets and online and couldn't find anything. We just had the traitor choose a monster he liked. I don't know how that part works.
The monster and everything that happens once the haunt happens is determined in the traitors and survivors handbooks. The traitors handbook instructs you on which haunt to use based on how it is triggered and there will be instructions for both sides based on each individual haunt.
Here are some simplified rules to help you get started better.
Monsters are never selected/chosen by a player.
It is unclear if you mean Haunt selection. When the haunt is triggered; as a result of failing a haunt roll after finding an omen; look at the first 2 pages of either the Secrets of Survival book Traitor's Tome book. A chart there will tell you which Haunt you have, based on the omen and the room. There is also another list there that tells you who is the traitor, based on which haunt you have.
One you have a specific haunt, the heros and traitor each have their own rules to follow. The traitor’s rules will generally go into which tokens are needed and how to set them up. No one is just selecting which monstors they want to use.