If you can find the GBDeflicker effect in Premiere Pro's effect list, make sure you haven't inadvertently selected on of the list filters.
If it still doesn't appear, make sure the plugin is in the CS6 Media core folder.
or on Windows...
If it's not there, drag a copy there from the plugin installation folder...
...on Mac in /Applications/GraniteBaySoftware64
.. or on Windows in C:\Program Files\Granite Bay Software\GBDeflicker64
If it is missing from the installation folder, then re-install the download.