How do I add a second mesh extender

Try powering off the first one that you setup, then add the 2nd one. once its added, try turning the first back on. 

I'd advise against daisy chaining them. 

Daisy chain: router---->extender----->extender

use them in star configuration

Star: Extender<-------router-------->extender

I also usually advise people to use a full mesh system if you're needing more than 1 extender. they tend to be faster and more stable.