Unfortunately the runtimes don't support IK yet, but they will be pretty soon. For now I'm afraid you'll have to go in and manually key whatever is affected by IK if you need it in a hurry. You can click the visibility dot next to the constraint under the Constraints nodes to enable FK editing while having the IK bone visible and then rotate your FK bone into position. It's not optimal I know, but once the runtime supports IK this won't be needed any more. The Unity runtime is probably one of the first to be updated.
