1. What restrictions are there on the use of OpenZinc?

OpenZinc is released under the Lessor General Public License. Details of the license are included with the code under the files COPYING and COPYING.LES or or from http:www.gpl.org.
2. What compilers can OpenZinc be compiled with ?

OpenZinc should be able to be able to be compiled with virtually any C++ compiler. A list of compilers and platforms that OpenZinc has been tested under are listed on the page Platforms and Compilers. Porting to additional compilers should be relatively straight forward.

3. How should I go about learning OpenZinc? 

As with any application framework much of the challenge of learning to use it is simply the relatively large code base involved. Frameworks are not meant to be used piecemeal but rather as a whole, in this case , as a complete solution to the graphic user interface. Fortunately, there is extensive documentation and numerous example programs provided with the code. The manuals Getting Started and Designer contain excellent discussions of programming techniques and programming examples.  The manual Addendum contains numerous code snippets illustrating various techniques. The directories /example and /tutor also contains numerous examples.