Not all themes have the log in link on the front page, so to log in, use this link;
http://yourblogname.blogsome.com/wp-login.php
To put a log in link on the front page, add the following code to your Main Page template:
<a href="{$siteurl}/wp-login.php">login</a>
If you want people to be able to register and post on your blog, add the following code to your Main Page template:
<a href="{$siteurl}/wp-register.php">register</a>