When you activate the MemberMouse plugin, MemberMouse's login page is used as the default login page in place of WordPress' login page. This provides you with an opportunity to style the login form and add additional content to the page. As a result, when you attempt to access the WordPress admin area at http://yourdomain.com/wp-admin (where http://yourdomain.com is where you've installed WordPress) you will be redirected to the MemberMouse login page instead of the WordPress login page.
You can still use the MemberMouse login form to access the WordPress admin area since MemberMouse integrates directly with WordPress' authentication system. Just enter in the admin credentials you've always used and you will be logged in and sent to the WordPress admin area.
You can follow the steps in this article if you'd like to use WordPress' login page as the default.