The example URL that you referred worked simply because your OPeNDAP server allowed direct access of the data source. By default, Hyrax (OPeNDAP server) doesn't allow direct access of the data source .
When your MCC web page says "OPeNDAP URL", I expected that MCC will utilize DAP2 protocol, not HTTP direct transfer. If MCC works with OPeNDAP URL, it should retrieve DDS/DAS and check conventions form there. This confused an avid OPeNDAP user like me.
Therefore, I'd like to suggest you to drop OPeNDAP from MCC . Simply say "URL for Remote File" or "Remote File (enter URL)". MCC doesn't utilize OPeNDAP's DAP2 protocol at all based on my testing with HPD  so far. If you'd like to keep "OPeNDAP URL", please improve MCC to use OPeNDAP client such as PyDAP instead of transferring netCDF data directly from a simple web server such as GitHub.com that doesn't provide OPeNDAP service at all.
If you don't get what I mean, please feel free to contact me at email@example.com
 http://docs.opendap.org/index.php/Hyrax ... ptional.29
 https://wiki.earthdata.nasa.gov/display ... t+Designer