In 0.3.0rc1 I have a six core reactor built 5x5x4, actively cooled and I get plenty of steam out, but I cannot get any power out, not even in the internal buffer. When it's actively cooled, does it cut the power output off, so only steam works?

That is correct. You will need to use the steam in some steam-consuming machine to get power; this is the trade-off of using active cooling. You get better performance overall, but you must have a more complex setup.

For small setups, you can pipe steam to TE steam dynamos. For larger setups, I recommend building a multiblock turbine. See my post on www.big-reactors.com for some basic info on how those work.

I'll be writing further documentation on the Technic Wiki this weekend.

So in the new version, if it is actively cooled u must use the steam to generate power?

Correct.