How to add power terminal pins in PCBNew in Kicad?
The power flags aren't placeable parts.  To place connectors for power, you should use connector components in the schematic for those connections.

Here's your schematic with power connectors and power flags:



The purple text by the power flags are the names of the footprints I assigned them.

Here's the completed layout:



Note that there aren't any components placed for the power flags although I did have footprints set for them in the schematic.



To repeat a comment I left earlier:
DO NOT turn off the design rules check.  It is there to help you.  When you turn it off, you downgrade KiCAD from an electronics design package to a drawing program that happens to have a library of electronics parts.

You can see this in your own layout:



That white line from R3 to C2 says that the trace you drew to connect those parts isn't really connected at one end.  If you make the board as is with that line still in place, you may find that there is a gap in the trace - no connection, circuit doesn't work.

You picked a nice, simple, common circuit to start with.

That's good.

Use this opportunity to learn how your tools can help you.  Turning off rules check and forcing a part onto the board from the layout editor worked this time - but you worked against your tools.  

Don't do that.
Let the program help you, and learn to work with it.  It will make things easier in the long run.