HI Jorge,
What do you mean you want to search by country?
I think the easiest way would be to add a filter for the country field in admin. The thing is with the countries plugin, you only save the 2-letter version of the country (for example GB for Great Britain, ES for Spain, SE for Sweden and so on): The other part of the field is only a display preference, so you won't be able to search for the field (also the name is localized, so if you are running on a spanish computer it will display everything in spanish). I think the best way would be to use a filter on the search page (like I said earlier).
Regards,
Andréas