How can I enable the digital signature option in the macros menu?

So I’m looking into how to digitally sign macros but nothing I do seems to stop the digital signatures option being greyed out. I can sign documents without any problems but macros just aren’t an option. I realise I could sign a document with a macro in but then that wouldn’t get through the trust settings.

I asked IRC, Nabble and google but came up with nothing. Does anybody here know?

And that done, do we know what kind of certificate you would use if you’re buying from a CA?

Thank you