In my DNS-327L the surveillance recordings are store internally by the system at:
\mnt\HD\HD_a2\Nas_prog\surveillancestation\videodata\
Only with system access you can get to those files.
The standard way to access the recorded files is creating a backup in the surveillance center application.
To do it you have to select playback and chose the recorded pieces clicking in the open record icon located at the top right. Then click backup and specify the folder name and location, the cameras and the start and end time of the recordings.
The system will create a new folder where you can find the avi files of the backed up recordings.