Bookmark PayPal’s canonical starting point for Mobile Payments Libraries, which contains links to detailed and up-to-date documentation on the SDKs for supported mobile platforms.
If you’re interested in developing an iOS application using MPLs, take a look at the sample code for Inquire, a very well-documented mobile, social, and local application that uses MPLs to process payments. [With regard to MPL integration, you may be particularly interested in the PayPalViewController class.]
If you’re interested in developing an Android application using MPLs, take a look at the sample code for YardSale, an application that scans QR Codes and uses MPLs to process payments. [With regard to MPL integration, you may be particularly interested in the ItemDetails class.]
If you’re truly ambitious, port Tweet Relevance to iOS, Android, or BlackBerry and use MPLs to handle making payments.