From b2evolution manual
This manual page is outdated. NEW PAGE: Log In in b2evolution v5+.
 The Login Page
You have to login in order to edit or manage your site. There should be a "Login" link on the main page of your blog. Clicking that will take you to the login page where you can enter your username and password.
Your username and password were given to you at the end of your installation of b2evolution. They can be changed in the backoffice on the Users & Groups tab.
If you need to access the login page directly you can do so at http://www.example.com/admin.php for b2evolution 1.8+ or at http://www.example.com/admin/ for b2evolution before version 1.8 (add "/admin.php" resp. "/admin/" to your base url).
 Issues With Logging In
A: In the backoffice under the Blogs tab, or in the file /conf/_basic_config.php you have set an URL with a statement like this:
$baseurl = 'http://www.example.com';
Cookies will be set only for this URL and its subfolders. So the trick is: use that URL to access b2evolution, not another one!
For local tests, use
$baseurl = 'http://localhost';
If it wasn't working, and then you found and corrected the problem according to what you see here, but it still doesn't work, then in some cases you may have to clear out your cache/cookies/temporary internet files before it starts working.
 Q: Logging in leads to an "Access Denied" page. What happened?
A. The login page leads to the Admin section (The backoffice or Dashboard.) Basic users like Demouser do not have rights to access this page. If you want them to be able to log in, give them Blogger rights. You can set that in the Users section. In Settings_tab -> General you can set the 'Group for new users'.
 Q. Can I have the login page be the default page for my blog so users have to login to view the blog?
A. The following forum post (1) has a discussion of using the login page in this manner.
 Q. How do I customize the login page?
A. Version 1.10.3: You can edit the login page by editing the files ../blogs/inc/VIEW/login/_header.php, login_form.php and _footer.php file on your server.