Astah User's Guide
Select the Sequence Diagram in the Structure Tree, then check off the [Frame Visibility] in the Propety view.
This is the tool palatte of Sequence Diagram, for all the common tools, check out here.
There are three ways to create Lifeline.
There are two ways to create Message.
Select [Synchronous Message] or [Asynchronous Message] from the Tool palatte and click target Lifeline.
Select [Synchronous Message] or [Asynchronous Message] from the Tool palatte and click Lifeline and then anywhere other than Lifeline.
Select [Synchronous Message] or [Asynchronous Message] from the Tool palatte and click on a diagram frame then Lifeline.
There are two ways to create Reply Message.
Select [CombinedFragment] on the Tool palatte and click on Diagram.
You can create Operand from the CombinedFragment's Property view or diagram's pop-up menu.