From WP forums ( http://wordpress.org/support/topic/plugin-wordpress-mobile-pack-incorrect-location-of-plugin-directory?replies=2#post-1654000 ):
Small bugfix for you. My plugin directory is not in the default directory and this plugin throws a fatal error.
On line 40 of mobile_pack_base/functions.php it should say:

$wp_mobile_pack_dir = WP_PLUGIN_DIR . '/wordpress-mobile-pack';

instead of

$wp_mobile_pack_dir = WP_CONTENT_DIR . '/plugins/wordpress-mobile-pack';

Review changes in defines after 2.6 to make sure we don't break anything: http://codex.wordpress.org/Determining_Plugin_and_Content_Directories