Well i've managed to make it work. When i enter absolute path for each file i want to minify then it works. For example:
http://www.yourdomain.com/wp-content/plugins/plugin/somefile.js
But if you want to do the same with relative path it wont' work. For example, if you insert this kind of path:
wp-content/plugins/plugin/somefile.js
Hope this helps.