Download an evaluation copy of Mathcad Prime. (Current version is 6.0) It will function as a full version for 30 days; after that (without a paid license) it becomes Prime Express. I'm not sure you can download Express by itself.
Which means go (e.g.) here: http://support.ptc.com/support/mathcad_downloads
click on the 'download' for Prime 4.
Fill in the form with your data.
You'll get a download link by e-mail for Prime 6.
Download and install the software. During the installation you can choose between a license for an evaluation period (30 days max) or install as Express immediately.