I am trying to find rim effect. I have attached a picture showing dialogue box of "add effect, offset, rim colour and mask colour" that I have seen being used, but I cannot find this option. I would be grateful if for any advice on this. Thank you.
What version of Blender are you using?
The stable version (Blender 2.79) doesn't have the "new" Grease Pencil, which comes with the rim effect.
To experiment with the new tools, you might want to try the version of Blender currently being tested: Blender 2.8 alpha. It is an alpha so it's not guaranteed to be stable nor will necessarily stay the same until the final release early next year.