How do I log kettle bell exercise??

Depending on how you are using the kettlebell as a piece of equipment while performing an exercise activity, you could log it under cardiovascular using "Strength training (weight lifting, weight training)" or "Circuit training, general" to get an estimate of Calories burned. Please note that the estimated Calories burned for cardiovascular exercises provided by MFP are based on published metabolic equivalent of tasks (METs), which are estimates for a general population and may differ for you as an individual.

The Compendium of Physical Activities has a new activity with an estimated METs of 9.8 for Kettlebell workout from a study comparing kettlebell workout with treadmill running.

You could use any one of the pre-loaded exercises with a similar METs value. The METs values provide estimates for a general population and an individual user's Calorie burn rate may be different.

METs - Activity

3.0 - Strength training (weight lifting, weight training)
8.0 - Circuit training, general
8.5 - Aerobics, step, with 6-8 inch step
9.0 - Running, 5.2 mph (11.5 min/mile)
9.0 - Running, cross country
9.8 - Running, 6.0 mph (10.0 min/mile)
10.0 - Aerobics, step, with 10-12 inch step