Hi there -- That "please wait" is a sign that something isn't quite right in the mailing or in the send test panel.
I hopped into your account and made a copy of the April2015 mailing. I was able to successfully send a test of that mailing after adding a subject line in the send test panel. If that's the mailing you're working in, adding the subject line there should get you going! Please let me know if you still run into sending issues. :)