You said the big download button... are you talking about the button on this site that will download the updater? If so, just click the download button and a zip file should appear on your desktop. Just to be clear, you get the updater from this link
http://www.eq2interface.com/download...rnUpdater.html
Inside the zip file will be 2 files. A read me file and a jnlp file. First, read the readme file. Read those instructions. But basically, just take the jnlp file, and place it on your desktop. then double click it (after you have ensure that java is installed, updated, and working). You can test your java installation at
http://www.java.com/en/download/installed.jsp
(hmm it would seem that there is an update that I dont have, they are currently up to version 12).
Anyway, once you have verified your java installation, double click the jnlp file and it will do the rest for you
It should auto detect your eq folder location. However, it has been seen in the past that some people have more than one installation that they did not know about. Make sure that the file path in the updater is the same one that you use when you start the game. Dont forget to read the update notes as well. A lot of usefull information is contained there.