Pages does not support loading LaTeX packages (they're not called plugins, but packages in LaTeX terminology). So you cannot just load up for example chemfig from CTAN that you mention.
Apple provides the following page describing the commands that are supported:
Pages does not include a full LaTeX environment, and is only able to render a subset of LaTeX commands.
For my own purposes I have used ordinary LaTeX rendered to PDF that I can drag-n-drop to Pages. As it is PDF it can be resized inside Pages without loosing quality.
In order to do that you need LaTeX installed. An easy way to get it installed is to install MacTex, which includes LaTeX.
You can download MacTeX from here:
Or if you have HomeBrew, you can use that to first install HomeBrew Cask and then install MacTeX using the following commands:
brew tap caskroom/cask
brew cask install mactex
For rendering to PDF you can use the pdflatex command or a GUI tool such as texpad.