Thank you for your post.
The Send Email script step allows you to send an email through your default email program. If you just want to open Outlook and a new message, you will need to use the Send Event script step, and then enter the Applescript to do this for you.
In the back of my head, I'm thinking, "Why do you want to open a blank email message? What are you actually trying to accomplish?"
In the Send Email script step, you can specify To:, Subject:, Message:, etc. That is, everything you want to send in an email. You can have these parts filled in by FileMaker fields. If you don't want this dialog box to pop up, you can check the option for this script to "perform without dialog".
Let me know if you need additional information, and if so, let me know what you are trying to accomplish. This would help in providing a proper answer.