I'm using Windows and had the same problem. For some reason PYTHONPATH didn't have the default directory for all my python packages listed.
At the top of your "post_test.py" file put:
import sys
print(sys.path)
That will tell you what PYTHONPATH is searching through for imports. Use ."append" to add the location of your steemapi folder. For me that was:
import sys
sys.path.append("C:\Python34\Lib\site-packages")
print(sys.path)