Guild Wars trial key is full of X's and doesn't work

If you look closely at the nearby text, you can see that the trial code has already been used on the account. Most of the code is X'ed out, because it now just serves to let you distinguish between different codes you may have applied at some point.

You don't need to enter the code again, you should be able to simply start up the game and play.

The serial code has already been applied to your NCSoft account. The reason it is full of X's is because they obfuscate full codes on your account details. Since the key is already applied, you should now be able to log into Guild Wars using the email and password you have chosen for the account.

The email will likely be the email associated with the NCSoft account (what you've scribbled out, though this may be different) and your password may or may not be the same as your NCSoft Master Account, depending on what you have chosen. You control the passwords for your Master Account and your Guild Wars account separately, so if you cannot log in using your NCSoft Master Account details, it's likely because they are different than your Guild Wars account itself.