i am just setting up the same DVR and was having the same issue - i found out that i wasn't initiating the playback properly due to the software being a bit "clunky".
When i go into the playback page and select any date or times there are no recordings found
What i have to do is click the "Search" button on the top of the playback page - this seems to index the hard drive and then show me dates and times where recording have occurred.
After the DVR has searched for recordings, the dates and times apprear green showing where recordings have been made and i can play them back