If the Umbrello build cannot find Qt with cmake then it seem likely that qmake is missing or broken.
  • Does
    Qt Code: 
    1. qmake -v
    To copy to clipboard, switch view to plain text mode 
    return a Qt version number and path?
  • Do you have the KDE 4 and Qt 4 development packages from your distro installed?
  • What is the exact error message?

The CMAKE_INSTALL_PREFIX in the install instruction refers to where you want the end product to be installed, not where Qt is installed.