Is this a fresh install of WDS of an upgrade from RIS?
Do you have an image installed on WDS?
see this document:
http://technet.microsoft.com/en-us/library/cc771670.aspxsection for installing image :
To add the default boot image included on the product installation DVD
In the left pane of the Windows Deployment Services MMC snap-in, right-click the Boot Images node, and then click Add Boot Image.
Browse to choose the default boot image (Boot.wim) on the Windows Server 2008 DVD, located in the \Sources folder.
Click Open and then click Next.
Follow the instructions in the wizard to add the image.
Repeat this procedure to add any additional boot images.
To add the default install image included on the product installation DVD
In the Windows Deployment Services MMC snap-in, right-click the Install Images node, and then click Add Install Image.
Specify a name for the image group, and then click Next.
Browse to select the default install image (Install.wim) on the Windows Vista or Windows Server 2008 DVD, located in the \Sources folder, and then click Open.
To add a subset of the images included in the Install.wim file, clear the check boxes for the images that you do not want to add to the server. You should add only the images for which you have licenses.
Follow the instructions in the wizard to add the images.
Now that you have a boot image and an install image on the server, you can perform a PXE boot on a client computer to install an operating system using the steps in the following section.