The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From National Parkinson Foundation, Inc.:
The National Parkinson Foundation has created the Parkinson's Toolkit.The Parkinson's Toolkit is the authoritative clinical practice guide for treating Parkinson's disease. This application includes everything a physician needs to know about treating Parkinson's. It covers the key issues in clinical practice; planning, symptoms, diagnosis, treatment, and billing information. The Parkinson's Toolkit is the most reliable and easy-to-use source for clinicians who treat Parkinson's disease. The Parkinson's Toolkit was made possible by the support of our sponsors and generous donations from people like you. To support this and other efforts to improve the lives of people with Parkinsons, please visit http://www.parkinson.org.