Ok, gotta clear something up – In your subject line, you mention “Remote App” … the WD Remote App is for the WDTV Media Players.
Do you mean the My Cloud app? I’ll assume you did.
So when you’re on your local network, the app should auto discover your NAS. When you click it, it will ask for user, then password, etc.
If you’re away from your NAS, you will have needed to generated an Access Code via your NAS’s User Interface, then do a Manual Connect, pick the device, and enter the access code.
The third method is to log in via the My Cloud service on the app – not the Choose your Device icons. There you would put in your registered email address and password.
Which method are you trying?