To install an app, click the Install button on the right side of the app name. Once installed, the procs defined in the app become available for use. The procs are pulled to a specific namespace (built from the app name and company name) and behave like native Vivado built-in commands. The new Tcl commands are automatically registered to the Help infrastructure.
Installing an App shows the steps. Xilinx recommends that you refresh the catalog (Refresh button) before installing an app to make sure that you install the latest version of the app (Step 1). Click Install to install the app (Step 2).
After the app is installed, the proc names become hyperlinks and the Install button is greyed-out. In Installing an App the designutils app has been installed.