You need to put the trailing slash in because 'Admin' is a directory and not a page.
If you want to be able to access the admin section from domain.com/admin you need to add a 301 redirect to your web.config file, e.g.
<!-- 301 Redirects -->
<location path="admin">
<system.webServer>
<httpRedirect enabled="true" destination="
http://domain.com/Admin/" httpResponseStatus="Permanent" />
</system.webServer>
</location>
Hope this helps.