File permissions for wp-minify plugin

toscho is right.
So you could probably give a try to another plugin. The Better WordPress Minify.

This one also uses a folder inside the plugin’s folder, BUT. You can change the folder and place it elsewhere. Please have a look at Advanced Customization.
screenshot http://s.wordpress.org/extend/plugins/bwp-minify/screenshot-1.gif

We have been using it without a problem.