Adobe® has stopped updating and maintaining Extension Manager (the full announcement can be found here, Jun 17, 2015). This means that starting from the CC2015 update (Jun 16, 2015) the Extension Manager application will no longer support the third party extensions installation process. And from now on, all my extensions (CC2105 and later) will be distributed via custom made, standalone installers. A short guides on how to install extensions for Adobe® Illustrator® CC2015 (and later) are available here: Windows, Mac OS X
Regardless of whether or not you have decided to upgrade to the latest CC2015, you can still use all previous versions of Adobe® products - CS6, CC and CC2014.1. And those applications are still supported by the Extension Manager CS6/CC. The problem is that the latest release of the Creative Cloud Desktop App (version 22.214.171.124 released on Jun 14, 2015 and updated on Jun 15, 2015) could break the ability to install extensions (using Extension Manager CC) for all previous CC (17.1) and CC 2014.1 (18.1) versions - extensions appear to be installed but some files are not copied into the correct locations. Some users have noticed this odd behavior, for example here: https://forums.adobe.com/message/7646202#7646202
To fix this you could try to reinstall the Extension Manger. Make sure all Adobe applications are shut down (Illustrator®, Photoshop®, Extension Manager, etc.), go to your Creative Cloud Desktop App, select Apps and find Extension Manger CC on the list. Click on it – a small settings icon should appear. Hit the Down-Arrow and choose Uninstall. Restart your computer. Once the application has been completely removed from your system, go back to the Creative Cloud Desktop App, select Apps, go to Previous Versions section and choose Extension Manger CC. Hit Install button. Now all your CC (17.1) and CC 2014.1 (18.1) extensions should install correctly.