Installing Smush Pro plugins

Hi, I can't get any new WPMU plugins to install. I keep getting an error about folder permissions being wrong but not direction on how to change them.

I'm still having issues with my server and WPMU (which various tech support guys know about) but this doesn't seem to be related (though I may be wrong). That issue usually manifests itself with me not being able to login in to the WPMU dashboard within my WP installation but on this site I am able to login but can't install plugins.

I've granted support access – could someone take a look?