The app's description on the playstore does not speak of "unzipping files to your sdcard". It rather seems to simply "handle" them, only temporarily extracting files to process them. Which would explain why you cannot find the files anywhere outside the zip-file itself (or outside that app).
If you want to extract files to handle them separately, you might rather wish to take a look at e.g. AndroZip Dateimanager instead, which is explicitly stating to extract files:
AndroZip File Manager helps you copy, delete, move, unzip/unpack, compress, encrypted ZIP supported, search, and organize your files, music, pictures, and folders just like you would do on your PC.