"I am at it' is grammatically incorrect but can be understood during conversation when referring to a particular place. For example, if you plan to meet your friend at the supermarket and your friend calls you asking where you are, you can say "I am at it", referring to the supermarket. However, a grammatically correct example would be "I am at the supermarket". Another use of "...at it" is for example: "The kids are at it again" referring to the children arguing, fighting or performing some other action once more.

I am on it is slang for I will attend to this problem or situation - I understand what needs doing and will deal with it. Not sure what "I am at it" means - possibly American English?