files path is not writeable

I have a multisite installation and I was checking your module. I make everysite to store its files inside sites/sitename/files to keep everything in order, but when I install your module I see:

    * El directori sites no es escribible.
    * Node Media upload path is not writable!
    * Node Media upload path is not writable!
    * warning: file_get_contents(sites/all/modules/node_media/README.txt) [function.file-get-contents]: failed to open stream: No such file or directory in /home/escolcat/drupal_upgrade/sites/all/modules/node_media/node_media.module on line 1237.

My "files" path is: sites/escoltesiguies.cat.conferencia_corea/files/

PS: I have checked twice: files directory is writeable (by IMCE and image modules)