Within the segment you've selected, go to the Communication tab and click New Draft.
In the Create New Draft dialog, select the Department, Project or Topic, and Segment from the respective drop-down lists, and click Create Draft.
This will automatically take you to the Email & Text page, where you can create a new draft message for members of a segment (the Recipients).
You'll see your segment listed as a group of recipients in the Recipients tab.
You can add additional recipients, or Preview Recipients in this tab.