If it says it does not match their records, then either your username or password is wrong.
Check your username by accessing the ADSL modem's admin page. It usually is at 192.168.1.1. type this in the address bar of your browser and press enter. The default username and passwords of various modems can be found online. Check the modem's manual or online documentation to find out where your username/password is stored. Username will be in plaintext but password might not be.
If the username is the same that you have been entering on MTNL's page then the password is wrong. By default the password is your CA number which can be found in your telephone bill, if it does not work then you will have to contact customer care regarding your login problem and they might reset your password.
If you have any issues regarding accesing the modem's admin page, post the exact model/brand name of the modem.
-edit-
there are 2 username/password combinations mentioned in this post. One is for the modem authorisation and other is your MTNL authorisation. Do not get confused between the two.