The download button opens the iTunes App Store, where you may continue the download process. You must have iTunes installed with an active iTunes account in order to download and install the software. This download may not be available in some countries.
From Kevin Abosch:
Lenka is a remarkably simple yet powerful, dedicated black & white camera app developed in France by world renowned photographer Kevin Abosch (kevinabosch.com) for professionals & amateurs alike, to create beautiful black & white photographs. With Lenka there's no searching through filters after you take the photo. Realtime B&W preview while Lenka's image processing with automatic exposure, yields photos with a distinct look and signature we hope you like as much as we do! Lenka is optimized for iPhones with 4 inch screens. For highest image quality, we suggest using an iPhone 5, 5c or 5s.