Hi Kate,
First, bravo to you for returning to school and challenging yourself with such a rigorous program.
Let's start with the accommodations. The Dean is 100% wrong in his take on this whole thing and obviously doesn't understand 1. ADHD and 2. The legal rights of students with disabilities. I'm hoping your school has a disabilities or support office with staff that understands your needs and what your rights are for securing support so that you can succeed in school.
If you have a document stating your have been diagnosed with ADHD, you are covered under the ADA (Americans with Disabilities Act) so that you are eligible for receiving reasonable accommodations. Some schools will go out of their way to help out students with ADHD, whereas others might only offer the minimum.
You can read more about your legal rights and possible accommodations at http://www2.gsu.edu/~wwwrld/BORWEB/accommodations.htm#question16
As for the medications..this is a very common question/dilemma. It can take quite a while to find the right medication and/or the right dosage. You'll need to work closely with your doctor to find which works best for you, as it's pretty much trial and error. Just don't give up!
Hope this helps-
Terry