There are 3 ‘core’ versions of .NET : 1.0 (1.0, 1.1, 1.2), 2.0 (2.0, 3.0, 3.5, 3.5 sp1) and 4 (4.0).
This means that if only the .NET 4.0 platform have been installed on your desktop computer, the current Xceed Zip for .NET won’t work since it needs the 2.0 .NET to be installed (or higher, within the same ‘core’).
I suppose that we will start targeting the .NET 4.0 platform when it will be popular enough. I have no roadmap right now. I will keep you informed right here as soon as I have new info.
If you want to know about the relationship between .NET 4.0 and .NET 3.5sp1, you can visit this link (at your own risks but it looks safe):