Here's what I've found for links about starting with openEMS. I'll update this as I find more info.

GitHub

The source code.

https://github.com/thliebig/openEMS-Project

The Forum

http://openems.de/forum/

There's a ton of info in the posts, but it can be very difficult to find. For one thing, the search function doesn't allow searches like "Gaussian step" because the words are "too common".

The good thing about the forum is that Thorsten, who wrote the software, is active and answers questions. I haven't felt the need to bother him yet, but I'm getting there.

The Wiki

Very useful in some places; spotty and link-busted in others, this seems like the best starting point.

http://openems.de/index.php/Main_Page.html

The Tutorials

There are a lot of examples here. Mostly RF and antenna stuff, which isn't necessarily applicable to what I'm doing at the moment.

http://openems.de/index.php/Tutorials.html