Luis,
To create an instance of a class existing in your tested .NET application (or in any .NET assembly), you need to call the class constructor. The class constructor name is _ctor(), but in order to make that name compatible with VBScript, TestComplete transforms the constructor name to zctor().
For example, the following code creates an instance of the CSalary class:
var CSalaryInstance = Sys.Process("UserApp").AppDomain("UserApp.exe").dotNet.UserApp.CSalary.zctor();
Please take a look at the note in the Calling Functions From .NET Assemblies Help topic - it contains information on class constructors. The topic also contains an example that demonstrates how to create an instance of the System.String class.
Still, we will improve the TestComplete documentation to make the information on creating class instances and the example easier to find. Sorry for the inconveniences.
Please let us know if you need anymore assistance.
Thank you.