I have no idea what I can learn from it. Any tips on using it to fix leaks?
It all depends...
First you have to be aware what kind of actions are considered leaks. This has nothing to do with HM3, it is only poker knowledge.
Once you have accumulated large sample of hand histories you can start evaluating your own actions, and learn about your tendencies in different situations. Are your actions considered leaks?
You can use HM3 during play on
poker sites that allow the use of it. It can help you to learn from your opponents, and to defeat them once you know how they act in most situations.
Hold'em Manager has been around for quite some time. Tons of learning material has already been written of it, and it would be counterproductive for me to repeat it.
Dive in! Search and learn, and become the skilled user that you wish to be.