The ADA does not cover the executive branch of the Federal government. The executive branch continues to be covered by title V of the Rehabilitation Act of 1973, which prohibits discrimination in services and employment on the basis of disability and which is a model for the requirements of the ADA. The ADA, however, does cover Congress and other entities in the legislative branch of the Federal government.
For additional information, take a look at the following resource:
Fact Sheet: An Overview of the Americans with Disabilities Act