That means the extension manager isn't recognizing your Dreamweaver installation.
if extension manager did not show dw cc 2015, just delete ExMan.db. location is:
/Library/Application Support/Adobe/Extension Manager CC/Configuration/DB/
C:\ProgramData\Adobe\Extenson Manager CC\Configuration\DB\
Other users have reported more success using the DWMXZone extension manager: http://www.dmxzone.com/go/22670/dmxzone-extension-manager-for-dreamweaver/
If all else fails you can install the extension command line and not use an extension manager:
Install an extension from the command line:
Navigate to the Extension Manager application folder.
(Windows) Enter the following command, specifying your own attribute value:
(ZXP format): ExManCmd.exe /install "c:\myfile.zxp"
(Macintosh) Enter the following command, specifying your own attribute value:
(ZXP format): ./ExManCmd --install "/myfolder/mysubfolder/myfile.zxp"
To specify a locale, add the locale command and specify the lang attribute before the install command, as follows:
ExManCmd.exe /locale "en_US" /install "c:\myfile.zxp"
Test the extension to make sure that everything works as intended