Plugin Installation Failures

I have successfully logged into WPMU, and no matter what plug in I try to install from within the Dev Dashboard I get the following error: Installation of Membership 2 Pro failed. Most likely reason for this are wrong folder permissions of your wp-contents folder.
I have no problems though installing plugins from other sources. I have also tried de-activating, deleting and reinstalling WPMU Dev Dashboard but the error continues.
Any suggestions?