Paolo,
On Windows signing kernel extensions is a thing for many years. It has been required to do since at least Windows Vista. On windows the kernel extensions come pre-built by VMware and are also signed by VMware.
On Linux however, the kernel is not exactly stable in its programming interface and as a way to help you, VMware provides the source of the kernel modules. So you compile these yourself. As such they cannot provide the certificates from VMware to sign.
Over time I expect VMware to handle this as well, by generating the internal/local certificates on your host and use those to sign the kernel extensions after you built the modules.
Long story short, this requirement for signing Linux kernel modules is fairly new and as such VMware does not have a "just click to upgrade" solution for this.
The admin guide I pointed to basically holds you by the hand on how-to do this yourself over about 5 pages (click Next at the bottom to continue)
I'm sure that there's some blog posts out there that explain this in more details as well.
--
Wil
