I want to switch to lyx, but I'm having a hard time trying to use environments. I red that they are supposed to be as a class in the drop down list at the top left (If I'm nor wrong, together with section, subsection, etc). But I don't see theorem or anything like that. I guess I can just use a piece of tex code, but I would like to use it properly. Am I doing something wrong or is there any problem with lyx? I'm using the 126.96.36.199 version for mac.
First you have to add a module. Go to Document > settings > Modules and choose one of the (several) theorem modules. See here:
Then you can access them from the environment menu: Note that a useful shortcut is to do "alt+p, space" and then start typing, e.g. "theorem". Press return.