can't install anything

Hi I am having difficulty installing plugins. I want to add Smush Pro and Hummingbird, but I keep getting the same error:
Installation of WP Smush Pro failed. Most likely reason for this are wrong folder permissions of your wp-contents folder.
Also, I am unable to update the WMPU dashbord plugin.
I have checked file permissions and both wp-content and plugins directories are set to 744!

Please can you help?