where do you find the WPA passphrase for MY NET N750?
By default there is no password.
When you setup the security option of your choice it will allow you to set the password you desire.
If your router is not brand new, default, use the reset button on bottom. Turn on router, let it boot up, press and hold the reset for 30 seconds and it will reboot. Once done rebooting, then you can start from scratch.
for reference, best speed will be attained from wpa2 personal. I would not use the wpa/wpa2 if possible, select the wpa2 only.