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You can refer the attached document for downloading the Mathcad Express.
Run the *.exe and you will be able to install the Mathcad Express.
Do remember that Microsoft .net Framework 4.0 is prerequisites for Mathcad.
You can download the files, from the below link: http://support.ptc.com/community/free-downloads.htm. You can refer below mentioned Q&A regarding Mathcad trial installation.
Where can I download Mathcad 30-Day Free Trial Edition?
A 30-Day Free Trial Edition can be downloaded from the link below.
Mathcad 15.0 Trial Edition http://www.ptc.com/engineering-math-software/mathcad/free-trial.
Mathcad Prime Trial Edition http://www.ptc.com/engineering-math-software/mathcad/free-download.
The Trial version is the same as the full commercial version of Mathcad with the exception that it will expire in 30 days after it is installed.