This is weird.
I installed e-newsletter back in December on my network, and activated it on two subdomains. It works fine on both of those. Here's the view from the WordPress dashboard when I mouse over "e-newsletter":
Note the red arrows I drew to the link url. Why are these urls not the same? This makes no sense to me. And obviously I can't use the plugin if I can't access it's dashboard/menu/etc.
I have experimented with changing themes, disabling all plugins, re-downloading and re-installing e-newsletter, even reverted to version 1.2.3 and then back to 1.2.4. What gives?