@Chaoie: The annotations list has two view. I think you are in the list view. Press the menu icon (the three lines) at the top right of the list. One of the options is the other view which should be "Notebook view". Select it and the selected text and annotation is shown.
Also, if you long press on items in a list, you will get a menu. On the annotations list, this will have "View details". That shows the full annotation in a new window. Press the arrow in the top left to get back.
If you do go to an annotation in the book, there should be a "Previous location" link in the bottom menu to return to where you were.
As you are a new Kobo user, some of the
UI conventions are explained here.