as already posted everybody can now play with Z-Wave SDK, as it has been completely released to the public.
To make it bit easier here small how to:
1 - you need to register on silabs.com first
2 - you need the Z-Wave SDK
3 - you need Keil Compiler License
4 - you need proper Keil Version
5 - you need to setup few things
6 - playing with examples
7 - ... and now you can compile your first project
8 - test your first z-wave firmware