A. Server 2008 is offered as "without Hyper-V" SKUs of Standard, Enterprise, and Datacenter editions. (No versions of Web and Itanium editions include Hyper-V.) The SKUs without Hyper-V don't include the Hyper-V role or the Hyper-V Manager MMC snap-in.
If you installed one of these SKUs and now require the Hyper-V role, you'll have to reinstall the server—there's no way to upgrade to the SKU with Hyper-V.
As a side note, in a best practice environment, the Hyper-V parent partition (the physical OS on the server) should be running only the Hyper-V role to maximize performance, security, and availability. This means that, in reality, you probably shouldn't be adding Hyper-V to an existing server running other workloads. Instead, you should perform a clean install then virtualize the other workloads inside a virtual machine on the new Hyper-V server.