I had the same problem--my Nikon d5300 came with WPA2 security already enabled for the WiFi connection, and I had no idea what the password was. I was connecting with an iPad, so I didn't have the option to use WPS.
Here is how I fixed it:
1) On the Nikon, go to Menu > Setup Menu > Wi-Fi > Network connection and enable the WiFi connection.
2) Wait a few moments for the connection to be established, then, in the Wi-Fi menu, go to "Network settings" and select "Reset network settings".
Doing this will reset the WiFi to an open connection, so you should now be able to connect to your Nikon without a password.
Once you have done this, you should open the Wireless Mobile Utility app and go to the Settings menu > WMA Settings > Authentication and change it back to WPA2 (which will prompt you to set a password) to prevent other devices nearby from accessing your photos.