HumanIK is basically a character setup that puts Inverse Kinematics across the whole body, a-la MotionBuilder control rigs. If you create one in Maya you get basically what you get in MotionBuilder, but with the added bonus of viewport selectable controls and the general Maya niceness for keyframe animation.
So to create a HumanIK rig in Maya, we have to make sure the plugin is loaded. If its not, you can activate it by going to Window > Settings/Preferences > Plug-In Manager. Then scroll down and check that mayaHIK.mll is set to Loaded. If its not there, probably best to check the Autodesk site for a download.
With that loaded, we can simply go to Skeleton > HumanIK... to load the HIK plug-in.
This will bring up a control box in place of your Channel Box. Basically we have the same thing here as in MotionBuilder, and it's pretty intuitive. Hitting either Create Skeleton or Create Control Rig will do the same thing with nothing in the scene, as it needs to make a skeleton to control to start with.
Hit Create Skeleton at the top, and one will pop up in your viewport. Then go to create, under the blue button, and hit Control Rig. This will build you some controls.
Voila! You can select controls via the panel, or in the viewport. Play around first, as HIK rigs work in interesting ways compared to our more regular animation rigs. The advantage here is easy importing to MotionBuilder, and the knowledge that the proportions are all exactly the same as the MotionBuilder rig.
Once you've animated a sequence, you can simply use the Send to MotionBuilder option in the File menu to switch it across. If you prefer you can export it manually, but the Send To feature tends to be the easiest way for me.
Hope this helps someone :)