FileMaker Server Requirements
This server-side plugin works with FileMaker Server 9 and up. It does not require Server Advanced. Your server needs Java 1.5 or higher, but most people are on 1.5 or 1.6 by now so this should not be a problem.
You can't be using self signed SSL certs on server.
Don't Have Server?
Consider hosting your FileMaker solution with a Zulu hosting expert.
Requirements in Your File
Check out the integration section for details on how to prepare your file for iCal publishing. You need to make some changes to your file (to import some scripts, etc.) but in terms of requirements, all you really need is a table with some dates in it. The "publish" script you run in your file to tell Zulu about your calendar for the first time requires FileMaker 9 or higher.
The privilege sets used by accounts logging in to the calendar from iCal (or from your iPhone) need to have access to the fmxml extended privilege. Remember, this is set at the privilege set level and may not be on by default.
To turn this on visit File / Manage / Security (or File / Manage / Accounts & Privileges) and Edit the Privilege Set you're interested in. You'll see "Extended Privileges" in the lower left. Turn on "Access via XML Web Publishing".
Finally, iCal won't let you use an account with a blank password.
This works with Apple system 10.5 forward. Older versions of the Mac operating system have older versions of iCal that don't support served data.
A special case: Multiple Machine Deployments of FileMaker Server
If you have FileMaker Server's component deployed on more than one machine, edit the script "Publish Calendar" you'll import into your file and find the comment toward the bottom of the script which begins "If you're using a Multiple Machine Server Deployment..." Follow the instructions in that comment to make a small change to this script.