Note that Dynamic C is really only useful if you're developing for Digi's Rabbit-brand embedded products. The compiler and debugger only work with that hardware -- you can't compile programs to run on a regular computer.
Digi does require that you set up an account and log in to download these files, and there may be export restrictions to certain countries. Where are you located?
If you have Rabbit hardware, which version of Dynamic C are you trying to download? I was able to request Dynamic C 10.72E (for Rabbit 4000/6000 processors) from this URL (click on "view"):
And Dynamic C 9.62A (for Rabbit 2000/3000 processors):
If you continue to have trouble, you'll have to email Tech Support for help.