Thanks for posting!
By default, there should be an Admin account that has full access. The username is Admin and the password field should be left blank. Are you able to log in to IWP with this account?
If not, you may need to enable the Instant Web Publishing extended privilege set for this account. To enable this, open the file in FileMaker Pro and head to File -> Manage -> Security. In this dialog, click on the Privilege Sets tab, select the [Full Access] privilege set and click the edit button. Then, enable the "Access via Instant Web Publishing (fmiwp)" option and click OK. This should allow you to log into this database using the Admin account via IWP.