First, you have to open an archive. Select Open Archive from the File menu or click on the Open speedbutton on the toolbar. Clicking on the small arrow button opens a drop down menu with a list of the last opened archives.

Now there appears a open dialog box like in many aother applications. Just select the file you want to work with an click open (if you don't want to work with an existing archive, visit the Create new Archive section)

Now you can see the files stored in this archive, their size, compreesion rate and path You are able to extract the files, add new ones, delete files or open files without manually extracting them.