What does this "Not Installed" symbol mean on the data sheet?
It means on their reference design/eval board it is not installed. That means if you think you need more capacitance, you can add one. Since TI parts are cheap a saving a few pennies is a big deal so they leave off the caps. The feed back loop cap (C8) is usually useful if you need to stabilize the feedback loop for any reason. 



TIP: if you have a question, it is good to poke around all the documentation you can find for that part (use the product page and any app notes they have, evaluation board files are always good to check), sometimes there is only a hint of more documentation in the datasheet or website. I'm not saying your wrong for posting the question, I'm suggesting a tool for future use.