Hi, Zorin 10 was based on 15.04 NOT 15.10!
Support for Zorin 10 ceased at the end of January this year, just like Ubuntu 15.04. You have to remember that any 'main' distro release will not have a 'forked' release at the same time - devs of 'forked' distributions have to look at how the main distro is put together and then look at how they can 'tweak' it to match the look and feel of the fork - this can take anything up to 3 months plus depending on the level of complexity/changes of the main distribution, plus how many devs are available to work on it. Apart from the Zorin brothers there are only 2 more developers employed by the Zorin project, so you can see it cannot compare to larger distributions. Be patient 12 will be on its way soon! My advice would be to drop back to Zorin 9 which is long term support until April 2019 - just like Ubuntu 14.04 on which it is based.