I’ve found a lot of addons for FSX required the .NET framwork 3.5
Sadly this doesn’t work on my Windows 10 machine so cannot use anything that requires it. Does this?
EditVoicepack XL does use a newer version of the .NET Framework, so my first thought is that .net 3.5 is not needed.
But to be honest I do not believe I have ever tried running it on a computer without .NET 3.5 enabled. As it is an integrated (though optional) part of Windows 10 it is somewhat worrying that you have problems getting it working.
I recommend you try the evaluation version available from the “try and buy” page before purchasing a license.