Files that I add don't compile

I'm new to Xcode 6, and I have a problem:

Files that I add (f.e. to emptyExample) don't compile when I select "Build"
And "Perform action/Compile File" has absolutely no effect.
Cmd-K on open files doesn't work anymore.

When I select ofApp.cpp or main.cpp "Target Membership" appear on the right pane, but not when I select my files.

When I select the target (top left panel) I can drag individual files to "compile sources", but when I add a folder, it has no effect. (Copy items if needed, Create groups or Create folder references don't change nothing)

Wit the big "+" button, I can also select individual files, but when I add a folder it has also no effect.