You will need to do as the dialog instructed. If you are using a Mac system, hold down the option key at the same time that you open the file. If you are using a Windows system, hold down the shift key while opening the file. You can then enter both an account name and a password (passwords are case sensitive) other than the ones specified in File Options in order to open the file with different credentials.
And this advise may come too late for you, but it's a very good idea to save a back up copy of your file just before you use Manage | Security or File Options to change how you use account name and password to open the file. That way, you can discard the current copy and revert to the back up copy if you find that you have somehow locked yourself out of your own file.
Thanks for the suggestions. I tried and noticed the account name belongs to the institute where i started using filemaker, and I really couldn't remember what the password is. Should I contact the institute?
the other thing I tried is to do the same to another file (copy as suggested), without closing it and checking the file option, i noticed the account is what I specified but the password is twice the length of what i used. Isn't this weird? something is going on and what is that?
FileMaker encrypts all passwords. As an added security measure, all passwords are displayed inside Manage | Security with the same number of bullet characters no matter how long or short the actual password.