So you’ve upgraded your ESX 3.x servers to 4.0 and you’ve upgraded your vCenter server, now you want to access the shiny new hot-add feature to upgrade some running server’s memory. Except you can’t, the feature is no-where to be seen. Something to bear in mind though, your OS needs to support hot-add, so you’ll need a Windows Enterprise or Datacenter edition.
Here’s how to enable it:
Upgrade the VM’s tools, if you haven’t already.