I'm just writing a little ACE-Calculation were I want to analyze the archive values of an input tag like cdt158
You usually don't need PI SDK for this purpose.
My development system is PI Server 2010 and PI SDK 126.96.36.1995. Dot Net version is 4.0 (for this I have created a file with name "PIACECLassLibraryHost.exe.config")
I am not sure if this is necessary. I am always leaving these things to PI ACE Wizard.
What's the Development Environment (i.e. Visual Studio 2008 Professional) you are using?
Although you need to add the .Net PISDK library which contains the collection PIValues, you don’t need to use .Net version 4.0. To troubleshoot you problem, I would first create a new project, using the original .net version which should be 2.0.
You might be adding the PISDK COM reference and not the PISDK .NET reference. If you are not sure, which one to add, my suggestion for you is to create an empty console application and add the PI SDK reference whose component name is "OSIsoft.PISDK". Add some code to this program just to show the name of your default PI Server. Run this application and check if the console application is using the PISDK library properly. If so, do the same steps on your ACE project to add this reference.
Let us know if you could solve this problem,