Mappings on this forum are posted either as an XML file, an XML file plus a JS file, or a ZIP archive with both an XML and JS file inside. If it is posted as a ZIP archive, you’ll need to extract that first (there are several ways to do this depending on your OS). If there is only an XML file, that is all you need. If there is both an XML and JS file, you’ll need both of those for the mapping. Put the XML file and, if there is one, the JS file in your user controller mapping folder.
Start Mixxx (or restart it if it was already running) and go to Options > Preferences (or press Ctrl + P). On the left side of the preferences window, look for Controllers and select your controller. If your controller is not listed on the left of the window, see the Troubleshooting page on the wiki. Once you have selected your controller on the left, select the mapping you just installed from the Load Preset drop down menu. It should appear at the top of the drop down menu. Check “Enabled” and then click “OK”. Enjoy mixing!