You shouldn't have to tell your employer that you are bi-polar. It is legally none of their business.
That being said...bi-polar people are often not consistent with their medication and it may be evident to your employer that you have 'problems' if you do not work closely with your doctor and take your medication faithfully. You have to treat your disorder as if it is merely another simple condition that you have to take daily medicine for, such as high blood pressure or diabetes. Statistics show that more often than not, people with bi-polar disorder do not consistently take their medication, which is the whole key to overcoming the disorder. If you are faithfully taking your medication and it's not working, talk to your doctor about changing it until you find one that works.
As a former friend to two different bi-polar people, I cannot stress to you how important it is that you take your medication and regulate your disorder. Whatever side effects you have from regularly taking your medicine are a small price to pay to help your friendships and working relationships remain healthy, and intact. For that matter, you doctor can help you fine tune your medication so that your side affects are not as severe or even noticable.
You need the relationships around you, and if you do not control your disorder you will alienate yourself, I promise. This will absolutely affect your ability to hold down a job and keep friends. Which is why you often see people with bi-polar disorder have such shattered lives. You don't have to live this way, if you commit to faithfully treating your disorder.