In the Workspace, double-click Message Grammar for the message you wish to add segments to. The Message Grammar window appears. Note that if you imported the MSH segment first and then created a new message, the MSH segment will already be present.
Click and drag the segment you want to add from the Workspace, and drop it into the Segment Grammar panel under MSH. In the screen below, the PID segment has been added to the message's segment grammar.
Repeat this process for each segment you want to add.
Segment Grammar Panel, Showing PID Segment
Remember that the order of the segments in each message is important. You can change the order at any time by dragging a segment to a different location in the Segment Grammar panel.