There is nothing wrong with "admitting" you may or may not have a learning disability ... it's still up to them to assess whether you're telling the truth.
But, before you go letting them diagnose you, you should make the decision as to whether or not you intend to believe their diagnosis. Especially with ADD, the medical community is very fractal. Many think that there is no such thing. Others see ADHD [the H is for Hyperactive] as being a true disability, and ADD as not existing. Others are more than happy to unecessarily drug you all up like HerbinMike. When you are taking the tests, think if you trust the medical practitioner, and if the tests they are doing seem like they will actually give knowledgable information to the doctor.
Myself, I've had some doctors I trust tell me that I have ADD, and had school psychologists tell me I was lying, that there was no such thing unless I was hyperactive. In short, the school tried their utmost to convince me not to declare it; I didn't, and I regret believing them.
This was in high school. I found out later that some of what they said were straight up lies. Essentially, they had a regulation problem with the school system. With a learning disability like ADD, you can still be intelligent, and make it to high level classes, albiet with poor grades. The regulations for learning disabilities aren't made for people like that - they are made for the mentally retarded, and dibilitating disabilities of a similar nature. Or, at least in California a few years ago this was the case.
Some of the lies I was told by the school:
..Colleges will see that you had extra time on your SATs, and judge it accordingly.
..Modifying your curriculum will force us to change your classes.
..We will have to take you out of any Honors, International Baccelaureate, or Advanced Placement courses.
If you're apprehensive about special treatment, don't be. If you're all excited about the drugs - remember that they won't have the same potency if you truly have a learning disorder.
So, make a conscious decision about your mental health. If you can, research it; you can make an educated choice. Don't treat it lightly.