When i went to installation ferret in my rails application then i have faced a problem that is look like this ERROR: While executing gem ... (Gem::FilePermissionError) You don't have write permissions into the /var/lib/gems/1.8 directory. i used this command for ferret install sudo gem install ferret
If you have any preference in using a search engine for your site, I would recommend ultrasphinx, much more stable than ferret.
http://blog.evanweaver.com/files/doc/fauna/ultrasphinx/files/README.html
You don't have enough info here to really help you. what platform, etc... Check to see who the dir belongs to 'ls -l'
Can you install any other gems?
Serge