Correct me if I am wrong. Are you creating a 64 Bit ODBC DSN? If yes, Create a 32 bit ODBC DSN and then use it to access the database. Also if you try to access the TNS entry directly in Crystal Report, it might not work as you would need 32 bit Client version of Oracle to do so.
You can access the 32 bit ODBC at the below location:
c:\windows\sysWOW64\odbcad32.exe