The instructions are very clear.
Follow the document named
http://www.easysoft.com/products/data_access/odbc-mongodb-driver/getting-started.html, it seems okay.
You will provide a SID for your mongodb, as documented in steps 4 and 5.
That sid will be for your listener.ora.
It can be any sid. You wll decide.
You are doing a DG4ODBC(DAtabaes Gateway For ODBC) based connecction there.
DG4ODBC is a new program that replaces hsodbc, which was used in earlier Oracle Database releases.
You may take a look at my previous blog post. It is about Oracle to Sqlserver , but you will get the idea and you will understand what is being done.