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)