To use either the DCS or the TCG, you must log in to with the same username and password that you used to download the software from RPGsuite.com.
When you created your account it looked something like this:
Then, once you logged in, you saw this:
This is usually your account name. Typically, your account name is the portion of your e-mail address before the @ sign. If your account name conflicts with an existing one, your account name will have a number appended to the end. Also, special characters like "!", an extra "@", etc, are likely to be omitted from your user name.
Finally, you can login using the same e-mail address you signed up for RPGsuite with as long as you only have one account with that e-mail address.