There are several possibilties to access this function. Either you use the main menu (Action->Open), or via the contextmenu (Open). Another possiblity, the easiest, is to click (or double click, if you've deactivated the "Hot tracking" option) onto an item of the zip content list.

It will automatically extracted to your temp folder (to set it, go to Options Dialog) be opened with its associated application, for example, a .doc file will be opened with Wordpad.

If you want to view the file with a special application, choose "View" out of the menus. As standard, the file will be opened with Windows Notepad, but you can also use any other program, e.g. a hexeditor. To set it, you have to select it over the Options Dialog.