There may be two reasons why you can't log into your Demo account;

1. Please confirm that the login details you have entered are correct and try again (passwords are case sensitive). If you continue experiencing issues, please contact Customer Services with as much details as possible (a screen shot of any error message would be good).
2. Demo accounts are available for 60 days, then they expire. If your Demo account was created over 60 days ago, then it has expired, you will need to add another Demo Account.

Here is an article on how to add another trading Account.