If you perform poorly on tests and wish to explain that to the committee, be sure to have a reason for your poor performance in hand (e.g., dyslexia, another learning disability, or not a native speaker or English). Simply explaining that you don't test well will not help your application as most graduate programs entail many tests.
Keep all excuses and explanations to a minimum -- a sentence or two. Avoid drama and keep it simple. Remember that this is your chance to present your strengths and really shine, so take advantage of the opportunity to discuss your accomplishments, describe valuable experiences, and emphasize the positive.