In order to use Athome Video to stream video from your home computer, you need download and install AVS server. Refer to how to setup the server.
You need pay special attention to the video folder in setting dialog. Make sure you specify the folder that contains all your movies files:

After you have downloaded AtHome Video from Apple's App Store and installed on your iPhone or iPod Touch, tap on the icon as shown on the right to start the program.

On the first page, you can add or modify a streaming server. For the first time use, you will need to set up your first server:

If you tap on "Set up your streaming server ...", following dialog will slide in. You need fill in the setup information of this new server. Those information can be obtained from your streaming server running on you computer.

To edit existing streaming servers, you can tap on the accessory mark as shown below:

Following dialog will slide in, to allow you edit each field of the server. Or you can delete this server from your iPhone/iPod.