Before you can add KMS for Windows 7 and Server 2008 R2 to your Server 2008 machine you first need to make sure the server is at least SP2 then install the hotfix KB968912.
Once your server is updated and you are ready to install KMS on your Windows Server 2008 box simply run slmgr /ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx but of course replace the X's with your Windows Server R2 KMS key. Notice that I said R2 there? Yes that's correct use your R2 KMS key even if the server you are installing KMS on is not an R2 server. If you do not use the R2 key your Windows 7 clients will not activate against this KMS server. After you run the /ipk command run slmgr /ato to activate the KMS server. If this is your first KMS server it's a good idea to go out to DNS and ensure that it registers in DNS and that there are no other rogue KMS servers listed in DNS. You can find these by opening the forward lookup zones and look for _VLMCS records.
If you already have a KMS server inplace and you simply want to update it for Windows 7 you simply need to run the slmgr /upk to uninstall the old product key then run slmgr /ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx to replace the product key with your R2 key. Once that is done run slmgr /ato to register the KMS server then restart the KMS server service.