You have two choices. There is light-based AO, and material based AO.

Light-based is easiest to set up, but will apply to everything in the scene. This can have some problems. First, it will slow down the whole render - how much depends on content. Second, the amount of AO will be the same everywhere, and you may find no single value that looks right everywhere. Third, it can produce artifacts which you'll have to chase down, particularly in any version of Poser before P7 SR3.

The other is material based, adding an Ambient_Occlusion node. This takes a bit more work to set up, and is perhaps confusing if you've never worked with nodes before. But it has the advantage of only needing to be calculated where you say you want it, and can be adjusted to different settings on different surfaces. The other problem is the AO Strength parameter on the Ambient_Occlusion doesn't do anything. That can be dealt with by hooking up another node to manage the strength.

If you have P7 SR3 or better, I'd go with light-based.

You'll want to adjust the Scene Ambient Occlusion Options. Be aware that the Max Distance and Bias values have units, so when you see a screen shot from somebody or they tell you values to use, and they don't tell you what there Poser Display Unit is set to, then you will not actually know what they're using.

I use inches. I set Max Distance to 48 inches. I set Bias to .02 inches. I set Num Samples to 7.

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