☆ Right-click Messages in the Orchestration View tab to create a new message, you have four choices for the Message Type property.

☆ Expand Multi-part Message Types and then choose Create New Multi-part Message Type.

☆ Name your multi-part message type then expand it so you can see its Message-part_1 member (Biz-talk suggested name)

☆ Set the Message Body Part property to True so that it acts just like a regular message.