One last question to be clear - Openfire is the back end (Admin panel) for Spark, right?
Openfire is a standalone Jabber-Server. You can build your own Jabber-Network with it. All users who want to use it can connect to this server using a Jabber-Client. There are many different Jabber-Clients, e.g. Psi, Gajim, Spark, …
When they login to our site, they can automaticalky have access to the IM without a download. Is that what should be able to happen with Openfire?
No, Openfire is just a Server. You will need a client to connect to it. But there are web-based clients, like the (commercial) Spark-Web, which is realized as a plugin for openfire.
Another (free) web-based client is JBother. It is written in Java and supports Java-Webstart, but you will need an installed Java JRE. Another way is a complete web-based client like Meebo.com. This one is free to use, but you can’'t install it on your own server.
In my opinion web-based clients are not the right way to convince someone from the benefits of the Jabber/XMPP-Protocol. Its better to write some step-by-step tutorials how to install an jabber-client, which is not that complicated.