If this is the first time you set up the network and you are unable to see video when you log in with your user name and password on your computer. You will want to log out and make sure you put a check in the box labeled "open all channels" before you log in.
If this does not work you will want to get the mini cd that came with your dvr, put it into your computer and install the dedicated viewing software. It may be labeled Netviewer, NVMS Monitor, DVRClient, DXclient, or many other names.
You will want to open the program after you have installed it and put your DVR's IP address, port number, username, password, and make sure you check the box labeled "open all channels" before you click log in.
This should take care of your problem, but if it did not then you have an issue with your graphics driver not being installed, updated, or your graphics card will not support viewing video from a dvr.