Imbalance should be used to describe procedures, regulations, laws, labour. E.g., one can say in a work environment, "I am executing the same job as you and producing more positive results than you but yet you earn a higher salary".
Unbalance should be used to describe issues of a more physical nature, e.g. "your tyres in your car is unbalanced therefore you are experiencing a rugged ride", or "there is an umbalance in our weather and climate conditions due to global warming".
Imbalance and unbalance are adjectives. Balance can be used as a verb, e.g. "balance yourself on your bike or you could have a nasty accident", can also be used as a noun. There are many variations depending on the related context.