The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From Hanno Rein:
No Brackets is a simple but powerful calculator app using reverse polish notation (RPN), also known as postfix notation.In reverse polish notation, every operator follows all of its operands. Thus, 1 + 2 is entered as 1, 2, +. Using a stack, one can perform complex calculations in an elegant way without the need for any parenthesis. With No Brackets, you can perform all basic operations such as addition, multiplication and exponentiation. It also includes all trigonometric functions, their inverse and hyperbolic counterparts as well as several mathematical constants. Furthermore, you can perform stack operations such as swap, copy, drop and clear.You can copy and paste numbers from and to the stack using the clipboard. The undo function allows you to go back in time if you made a mistake.