Hey there MMoore,

It's starting to look much better, I've highlighted your problem with the red circle, for some reason this edge will not join. To get this view yourself go to custom polygon display and click on the border edges button, set the edge width to what ever is good for you, I have it at 4 (you may need to click on display/polygon components/border edges too, but I'm not sure).

To sort the problem out I would delete the 2 faces that I've shaded green and use the append poly tool to rebuild them, you will be able to convert to sub-d after that.

take it easy,

Mat.