When building a .NET assembly, by default Plato will build for .NET 2.0 if this version is installed on the PC.
If only .NET 4.0 is installed then by default Plato will build for .NET 4.0.
If both .NET 2.0 and .NET 4.0 are installed and you do not want to build for version 2.0 then you can either
a) configure your project to build for .NET 4.0 using the Project Properties dialog or
b) configure Plato to always build for .NET 4.0 by adding the following kind of data to your Plato.ini...