Question about using multiple dictionaries on the Mac

Goodmonrning everyone,
I though that a couple or 3 days ago someone wrote about dictionnaries, but I cannot find the topic, so I created this new one. Inthis post or article, I don't know, which I read, someone talk about app diccionaries for Mac: He told that it could have to down load the app the first time, and afterwards, YOu can add new dicitionnaries.
Firest at all, I would like to say that I really love this app: I work with languages (I'm translator) and so diccionnaries for me are really important, they are the bassis of my job. IN adition, these diccionnaries which Apple offers us are really very good. there are 2 different kind of diccionnaries: bilinguals as this person toalk about, and monolinguals, and they are really very well done. However, I have a problem: At the very begining, when I got start with my Mac, a friend of mine help me to down load the app, and he installs me the diccionaries which I need in this momen. However, now, I need another one, but i don't know how to download new dictionnaries, as I say, I have just the app, and some diccioanries installed, I only what to add a new one, monolingual english diccionary. So, someone knows and could explian to me, please, which steeps should I follow?.
Many thanks in advace:
María