Well to start with 1.15.1 is old and you should upgrade to current. I don't even know how you would have downloaded and installed this old version. You can re-download the distribution archive from osticket.com/download
If you are looking at Admin panel -> Dashboard -> Information and seeing a white ! in a red triangle then it means that you do not have the extension enabled in your PHP. Since there are so many flavours of Linux we cannot really tell you how to install the the zip extension.
You may find this link helpful:
Generally on Linux OSes you would install the php-zip package last I checked.