A total Linux newbie would get as far as downloading the demo.. finding it doesnt work and moving on.
There is no Linux demo to try, so the user would already have bought it and would already be committed and determined to get it to work. Their first port of call would be the forums where there are plenty of installation guides. I would imagine that most people would have tried the Windows demo or done some research before buying a product they can't try first.
However, I did note that the system requirements for the Linux version only state hardware OpenGL and nothing about the possibility of needing to download additional libraries.
On the mac I had to go to apples site.. register as a developer so I could get whatever the heck it was I needed to get Max to work
The only other thing that needs to be installed is X-Code, which although not installed by default, does come on the OS-X DVD. Therefore, there is no need to register anywhere for anything to get BlitzMax to work.
On another note, I'll be installing 7.10 later on today after an update for 7.04 killed my 3D acceleration and my complete failure to get it to work again! Wish me luck!