Adobe have discontinued Extension Manager (please see here for details) and, according to this page, it should no longer be possible to use Extension Manager to install the .zxp version of the plugin in Photoshop CC 2015.
Note:
In Photoshop CC 2015, you can no longer install add-ons directly using Extension Manager. Instead you must use the Creative Cloud desktop app.
(Incidentally, contrary to this information, I have been able to install the .zxp version of the plugin in Photoshop CC 2015 using Extension Manager CC v7.3.2.39 on PC.)
Instead, visit the plugin's web page on the Adobe Add-ons site: https://creative.adobe.com/addons/products/2465
... and click the 'Free/Install' button.
Make sure that you have File Sync enabled in your Creative Cloud desktop app ('Tools -> Preferences -> Creative Cloud -> Files -> Sync').
This should work fine (it has been tested with the latest version of Creative Cloud desktop app v2.2.0.129) but if you have any problems, just download the CS2, CS3 and CS4 .zip version from the plugin's support page and follow the manual installation instructions. The resulting files in the 'Presets/Scripts/' folder will be exactly the same as if installing the .zxp version. Only the packaging of the files differs between the two.