I agree, use FK rather than IK. Otherwise, I imagine you need to disable the IK handle through the... (if I remember correctly) Modify > Evaluate Nodes > Disable All.
Then, you can rotate the finger joints and right click on them and Set Preffered Angle. Then, rotate them back to where they were and enable the IK again. It should bend the way you want.
If not (or even if it does) I'd say use FK.