Step 1. In the dashboard click 'LISTS' and choose the list you want to open. Click 'view' to open the list. 

Step 2. In the list settings area click 'CREATE SEGMENT' to continue.

Step 3. Adding your conditions
First, name your segment and then start to set your conditions.
Choose +Mailed, Opened, Clicked Conditions. This will add a condition statement where you can choose from the drop down and fill in Have Not Opened and then fill in the time frame. For this example I chose 180 days. 

Once that is done click  'SAVE SEGMENT'. 

Thats it! You're done creating an Opener Segment. That was easy wasn't it?