We have recieved many questions about how to create Messages in Communication (Collaboration) Diagram.
The key is that you have to connect two Lifelines (Objects) with "Link" first. So let me show you how it works..

1. Select [Link] from Toolbar and create a Link between Two Lifelines (Objects)













2. Select [Message] from Toolbar then click on a link line. A red arrow appears, move mouse sideways to change the direction the arrow should point.




















3. Click one more time to determine.









Best regards,
Satomi