why I can't download QT 7 Pro

I paid for Quicktime Pro 7 but cant figure out how to download it. Why is this so hard?

Because it is impossible to download something that does not exist. What you purchased is a key that unlocks the "Pro" features that are already built into the free QT 7 Player app. If you do not already have the free QT 7 player app installed in the Utilities folder of your system AND you are using Mac OS X v10.6 or later, then download and install the "special" QT 7 player version made to run under Snow Leopard, Lion, Mountain Lion, and/or Mavericks HERE.

Quicktime is not in my System preferences in the apple menue.

You appear to be following the old instructions. With the release of QT X under Snow Leopard and the ability to install both QT X and QT 7 player apps, the QT 7 aux preferences entry via the "System Preferences" window was removed in favor of a return to the older, direct entry to the preferences window via the QT 7 player app itself. For current QT 7 installation instructions start HERE. For current QT 7 Pro registration instructions start HERE.