Just spent an hour trying to get an app to compile using my MdhXmlEditor class. Although I was declaring the header file for the class in the header file for the main window, it couldn't find the class. Eventually I discovered that I had to manually add the header and cpp files for the two libraries I'm using to the QT .pro file, like this:

# -------------------------------------------------
# Project created by QtCreator 2010-03-10T14:41:22
# -------------------------------------------------
QT += xml \
    xmlpatterns
TARGET = createImtFiles
TEMPLATE = app
SOURCES += main.cpp \
    mainwindow.cpp \
    ../imagemarkuptool/trunk/projXmlEditor/mdhxmleditor.cpp \
    ../imagemarkuptool/trunk/projXmlEditor/mdhxmlhighlighter.cpp
HEADERS += mainwindow.h \
    ../imagemarkuptool/trunk/projXmlEditor/mdhxmleditor.h \
    ../imagemarkuptool/trunk/projXmlEditor/mdhxmlhighlighter.h
FORMS += 

There's no equivalent of Delphi's "Add file to project", but obviously the pre-processor needs to know about this stuff; it can't rely on the compiler to find all the header files.