If you are getting 404 errors then it is due to the .htaccess file. The first thing is to make sure you have the rewrite module enabled then you have to make sure that .htaccess is uploaded on your server. If you don't see the .htaccess file then it means that the file is hidden (happens on Mac) but don't worry you can download the file below. Once you upload it to your server, rename it from APP.htaccess to simply .htaccess. After that check to see if the issue has been fixed. If not try the following solution.

If you have installed this script in a sub-folder (e.g. /public_html/short/)

1) Open .htaccess
2) Add  Options -MultiViews  at the first line
3) Add the RewriteBase rule, which in this example is short, but you have to change 'short' by the name of your folder: RewriteBase /short
4) Add a slash in front of all files (see example below) - In the example below, you should add a slash in front of index.php

RewriteRule ^([_A-Za-z0-9-]+)/?$ /index.php?action=$1 [QSA,NC,L]

5) Your .htaccess should like something like this

Options -MultiViews 
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f

That is it. If you have done everything correctly, then everything should be working. If it doesn't feel free to open a ticket but make sure to leave your ftp info so I can have a look at it.