The Download Now link directs you to the Apple iTunes Store, where you can continue the download process. You must have an active iTunes account to download the application. This download may not be available in some countries.
From Wolfram Group:
Remember the Star Trek computer? It's finally happening - with WolframAlpha. Building on 25 years of development led by Stephen Wolfram, WolframAlpha has rapidly become the world's definitive source for instant expert knowledge and computation.
Across thousands of domains - with more continually added - WolframAlpha uses its vast collection of algorithms and data to compute answers and generate reports for you.
Parts of WolframAlpha are used in the Apple Siri Assistant; this app gives you access to the full power of the WolframAlpha computational knowledge engine.