Do you have the latest version installed? I thought I had fixed this issue.
The problem is that the wizard didn't create the email include files necessary to send the email. To correct the problem at this point you can:
1) Create a new page (call it temp.php you will end up deleting it later)
2) Apply the Send Email Message server behavior (use dummy content)
3) Upload the webassist/email folder that will now have all of the necessary files
4) Delete your temp.php page, you don't need it anymore