I am able to access postfix admin via:
http//localhost/postfixadmin/login.php
but I am unable to access:
http://localhost/postfixadmin/admin

I followed the instructions perfectly, the only thing I wasn't sure about is the hostname variable in:
/etc/postfix/main.cf

Also, how can I test to see that the mailserver is configured properly. I would like to be able to send a test email or something. Thanks.

The hostname in main.cf should be the hostname you got from your ISP.

What is the error you get when accessing http//localhost/postfixadmin/login.php

I don't get any error with that URL, only when I try to load postfixadmin/admin. However, when I try to login via postfixadmin/login.php I am unable to.

I've tried the default values of admin/admin, and I've also tried changing the password in .htpasswd via your instructions, but neither works, seems something is wrong with the authentication.

Sorry, have the wrong url, you first have to create users to be able to access http//localhost/postfixadmin/login.php
You need to create them in http//localhost/postfixadmin/admin/, you should get the browser to respond with an authentication request where you should use the admin/admin combination ?
Do you have the AllowOverride All in your httpd.conf ?

Yes, I have allowoverride set to all in my httpd.conf file. Do I need to add something to my .htaccess file?

your .htaccess file should be in postfixadmin/admin/ and should have the content as described in the documentation. Be aware of the pathnames used in there, they might be different in your setup...