Error activating E-Newsletter

Everytime I try to activate the E-Newsletter plugin, I get this message:

There was an issue determining where WPMU DEV Update Notifications is installed. Please reinstall.

Only Akismet and WPMU Dashboard is activated.

WordPress 3.3.2

E-Newsletter 1.2.0

Farms theme: Falling Dreams 1.3.6, customized to fit clients style