Ok, so I followed the tutorial pretty much to the letter. I was near the end of video 2. I deleted the bottom face and extruded it together with Clipping and Merge like he showed, then proceeded to put in the other missing faces on the left and right of the base as oriented in this photo. When I showed the Sub Surf Modifier, I got this instead of it being completely smooth, you can see the weird flat part at the bottom. I'm not sure how to fix it.
One note, I did have an older version of Blender, 4.6x. I just recently updated it to 4.79 yesterday. I just tried to fix the bottom and I still get this problem. Please help.
Here's the .blend file: