As of right now the demo is very very popular and navigating to the website may be a little laggy. The demo can be downloaded from the current minecraft.exe build. It will last 5 minecraft days [1 hour and 40 min]. Good Luck! Have fun with the demo!
Note: The Developer(s) of MCDzienny have resigned and the software is no longer supported. Errors are bound to occur and Mojang has not attempted to resolve them.