Can't open downloaded stack on OS X

[Dar Scott]

I downloaded a stack from the Internet (Altuit's cool XMLtree.rev), 
but I can't open it with Revolution.  It is dimmed in the open 
window.

I had in the past downloaded Galactic Conquest (stuffed?, I don't 
remember) and simplechat (zipped).  I was able to open those stacks 
after uncompression, but I don't remember what I did.  I would have 
thought the zipped stack would have the same problem as an 
uncompressed stack.

What am I doing wrong?

Dar Scott