That update is downloaded automatically if you have that option set. Here's what MS says:
"To have the newest versions automatically delivered and installed as soon as they are released, set the Automatic Updates feature to Automatic. The version of this tool delivered by Windows Update runs on your computer once a month, in the background. If an infection is found, the tool will display a status report the next time you start your computer. If you would like to run this tool more than once a month, run the version that is available from this Web page or use the version on the Malicious Software Removal Tool Web site."
To do it manually download it from this site and run it: