Stable releases and documentation for each of the SDKs are available at the following links:
The Liferay Mobile SDK is a framework for building native mobile apps that integrate with your different Liferay Portal instances and their portlets. The SDK provides the means for your mobile apps to easily consume Liferay Portal's core web services and the web services of your custom portlets. It wraps Liferay JSON web services, takes care of authentication, makes HTTP requests (synchronously or asynchronously), parses JSON results and handles server side exceptions.
The Liferay Mobile SDK is compatible with Liferay Portal 6.2 and later, and it comes with the Liferay Android SDK and Liferay iOS SDK ready for you to download and use.
If all you want is to access Liferay's core services from a mobile app, follow the links above. However, if you want to generate services for your custom portlet, read the SDK Builder documentation.