JWT_PRIVATE_KEY is the path to the file that contains your private key. Here is the documentation on generating a RSA private-public keypair. 

JWT_PRIVATE_KEY_PASSWORD is the password you set for the file that contains your private key.

This did not help me.  Can you please clarify how you get the value for the JWT_Private_Key_Password.  Below is the code snippet from GitHub for the C# SDK. I have not been able to find out how to get that value as it is only a string.  From what I can tell there is no password for the Private Key.  Please help.  Thanks

var boxConfig = new BoxConfig(<Client_Id>, , , , , );

var boxJWT = new BoxJWTAuth(boxConfig);