The one thing my university does a lot is offer these students the ability to write their exams outside of the normal area (eg the PAC with like 1000 of your closest friends writing with you) in an area set up to minimize distractions.
Also, in these areas (since they are the only ones writing there) studentss are allowed to get up and walk around if they feel the need to, I think.
Also available are councellors who can assess and advices these students.
There are probably other considerations too. I don't have ADD so I don't usually have much to do with the Services for Students with Disabilities area.
