2 bone IK isn't currently possible. This is because 2 bone IK can be solved analytically (there is one correct solution), while >2 bones requires some sort of integration (there are many possible, valid solutions so the resulting pose depends on the poses of previous frames). Currently posing a skeleton is deterministic in that it does not depend on previous frames. For a given animation time, the pose will always be the same. This is convenient for many reasons, but we do plan to allow non-deterministic features eventually (for physics, >2 bone IK, etc). This means you can get different poses based on factors like how often and how many times the animation was previously applied.
You can use the Pose tool, select any number of bones, and then manipulate them using IK. Note this is for manipulation only, not for interpolation during an animation.
As Erikari mentioned, path constraints may be an acceptable workaround.