You can easily define usernames and passwords in web.config file:
Code:
<authentication mode="Forms">
<forms>
<credentials passwordFormat="Clear">
<user name="Iceman" password="IceIsTheMan123!" />
</credentials>
</forms>
</authentication>
Unfortunately, there is no way to assign a user to a role in web.config. However, you can always assign a user to a role in code if you need to -- like in a Page_Load method, if you wanted to test something out, for example.
In the long run, it might be better to try to get your regular authentication / authorization working right. Are you not able to log in the user at all? Or is it just not picking up on the role?