Show post exerpt on category page (Shamrock theme)

Hi to everybody!

I am quite new with wordpress.
I need to show posts excerpt in the category pages. I saw I am not the first having this problem. I tryed for two days to do it following the tips of this forum but I can’t! When I check the folder of my theme there are no files named as here in the forum. Others does not look like as showed in the examples I saw… The theme i am using is Shamrock
the url of the web is ontravelwriting.com (it is still a bit messy…)

Thanks a lot,