Trying to install WSUS on Server 2008 but it's not listed in Server Roles

you might need this KB update: kb940518 Update

Update for Windows Server 2008 Server Manager (KB940518)

Instructions

To avoid a restart of the computer, verify that Initial Configuration Tasks, Server Manager, or servermanagercmd.exe are not running during the installation process. You must also log on to the computer as an administrator to install this update.

1.    On a computer running Windows Server 2008, download the Update for Server Manager in Windows Server 2008 (KB940518) package from the Microsoft Web site.

2.    Double-click the downloaded package to start the Setup wizard. Follow the instructions in the wizard to complete the installation.

3.    Open Server Manager from one of the following locations:

·         The Start menu

·         On the Start menu, right-click Computer, and then click Manage

·         On the Start menu, point to Administrative Tools, and then click Server Manager

·         Quick Launch toolbar available on the Windows taskbar

Note: If WSUS 3.0 SP1 is already installed on your computer, skip the remaining steps.

4.    On the Server Manager home page, in the Roles Summary area, click Add Roles.

5.    Select Windows Server Update Services from the list of available roles on the Select Server Rolespage of the Add Roles Wizard.

6.    Follow the instructions in the Add Roles Wizard to complete installation.