I found MH doc page saved in my computer recently. And I found this..
Loop Animation
Create a loop of the action between two selected time markers, by blending the keyframes in the beginning and end of the loop. This is useful e.g. to create walk and run cycles for games. For good results, the poses at the beginning and end of the selected region should be similar.
Blend Range: The number of keyframes used for blending.
Loop in place: Remove the X and Y components of the root bone's location.
Loop F-curves: Loop the animation.
Repeat Animation
Repeat the action between two selected time markers. The actions should preferably be looped before it is repeated, to make the beginning and end match seamlessly.
Repeat Number: The number of repetitions.
Repeat F-curves: Repeat the animation.
After try and try actually it simple. This is what I do..
1. Bake animation to frames I want to make loop. For example from frame 20 to 60.
2. Mark the start and end animation and
make sure both selected.
3. Set bend range (exp: 5). Check loop in place if want to loop in place.
4. Pres Loop F-curves button. Viola! Here it is a loop animation.