You may need a current diagnosis in order to take the test with accommodations. The following articles have information about educational testing for adults:
Check the GED website to find your local GED Testing Center. The site also offers information on available accommodations for candidates with disabilities. The page also provides information on how to request them.
There are several national organizations that can help you through this process and provide referrals to local professionals. You might want to contact the International Dyslexia Association or the Learning Disabilities Association. In addition, you can look in your local phone book for “educational testing” or “psycho-educational testing”.