Ok first of all, mining is no longer possible with Steem, and these instructions are also out of date in a couple ways.

 It may point in the general direction of successfully building still, but understand that if you want to build a node now, you should do a git checkout of the latest release tag (**not `master`**) before doing the git submodule update step.

The witness_node_data_dir is created after running steemd for the first time. So run it, give it a second, and then quit it with CTRL+C. Then there will be that directory with config.ini within.