There are three ways to access search on the Kobo Mini. To search your library or the Kobo bookstore, do as Filmo suggests: select "Search" from the menu that appears when you touch the menu key (the four horizontal line sin the upper left corner) of the home screen.
But I rather suspect you're actually looking for "search within the current book." That's easy and works well. To find other occurrences of a word on the current page, simply highlight a word, and the third icon from the left in the menu that appears at the bottom of the screen (the magnifying glass icon -- which is not a "make image bigger" button, as it is in some apps, but is "search for like a detective" icon).
To find a word that's NOT on the current page, bring up the in-book menu. On the bottom of the screen you'll see an icon of an open book. Touch that. "Search in Book" is one of the choices.
