Note: If using Mac 10.7 (Lion) or 10.8 (Mountain Lion) you need to go to Java Preferences and check the Enable applet plug-in and Web Start applications checkbox before logging into https://cuvpn.colorado.edu, as shown in the image below.
To open Java Preferences:
Open a Finder window.
Click on Applications located on the left sidebar,
Open the Utilities folder.
Double-click Java Preferences.
Log in using your Identikey username and password.
In the Client Application Sessions section click the Start button.
I tried to access my custom VPN but got the standard version
Make sure to press Enter, or the "Go" button, in the "Sign In" line of Network Connect after entering the custom VPN URL. Network Connect acts like a mini-browser to get to the actual sign-in page, and you have to tell it to go to the URL before you can login there.
Read more about connecting to the network with a Custom VPN.