Hi @Gaetano ,
First of all welcome to the group!
If you'd like to search for a specific keyword on a comment, you can go to the search bar in Trello and type comment:typekeywordhere.
That will show you the card where the comment appears. While the comment won't be highlighted, as a workaround, you can click on CMD+F on your keyboard to open up your browser's search bar. Then you can type in your keyword and you'll be able to find it.
You can check this article to see other search locators that can be used in Trello: http://help.trello.com/article/808-searching-for-cards-all-boards
When I use CTRL-F in the web browser version of Trello and search for a word used in a comment on a card, it does not work; it only works when the card is opened.
When I use Jump To on the upper left side it finds the words I search for and shows the cards where it is found. However, If I just type a part of the word, it doesn't work either: how can I search for that, using for example an *?
@Patrick van Hoeijen if you want to find words in comments on all cards not just those you have open, you can use the "comment:" modifier in the search feature: