What do you do with OpenSteer? Is it just for testing your AI code within the demo application or is it a library for own code? Do you know a good tutorial on how to use it in your game?

OpenSteer is a steering behaviors library.

I've never used it, but checked out the demos.

The best tutorials, in my opinion, would simply be to check out the code that comes with it, there are plenty of examples. The demo application you're talking about is exactly an example of an application using the library, from that you can probably figure out how to use it.