The trial version of the program is fully functional, and you may use it as long as you like. However, any output of the program (printed pages or images) carry the message that this is the trial version. Of course, I want you to buy the standard program.
You must be running a version of Microsoft windows or a Mac operating system. Mathpad is a Java program and so you must have a Java Runtime Environment (JRE), version 1.4 or later. If your computer is fairly new, you undoubtedly have an appropriate JRE. If you're not sure, search for a folder named Java on your computer. Mathpad does run more smoothly with version 1.6.
When you click on one of the links below, the program is downloaded. Probably you'll want to save this download somewhere, say your desktop. After the download, a double click on the file will install Mathpad.