Go to Marketing Activities.
Select your Email and then click Edit Draft.
In this example we're making the Subject Line dynamic. Click in the Subject field, then click the Make Dynamic button.
You can also make an element inside the email dynamic. To do this, select the area, click the gear icon, and select Make Dynamic (or Replace with Snippet, depending on what you're doing).
Enter the Segmentation name, select it, and click Save.
Your segmentation and its segments appear under the Dynamic tab on the right.