I do not know zurb, just Ruby as a language/platform, and I think that if that first 'require' throws, then most probably some paths are not set up properly in your server environment.
Is Zurb distributed as a gem? Are other gems found well and are require'able by your code? If not, or if you don't know, try adding
require "rubygems"
as the very first line of your code (well, after some initial shebang and other comments). It should normally be invoked by the framework, somewhere before your file, but as I already said, I don't know Zurb. Maybe it simply does not do that for performance reasons.
Also, run some simple file in the same environment and inspect the $:
variable, it should show you the paths that are searched for modules. If in very deadline'y times, you can even temporarily push a new path to it (just replace the '.' with your target path), but it'd be best left as a last resort. Your zurb module seems to sit in Gems, so it should be found automatically.