After installing the MQTT plugin, return to the Install new Plugins... window and search for "MQTT Subscribe". Click Install to install the plugin.

After the installation finishes, a Plugin management log dialog box will appear letting you know that you need to restart OctoPrint to complete the installation. Click Restart now to restart OctoPrint. When you're asked if you're sure that you want to restart, click Proceed.

After OctoPrint finishes restarting, you'll be prompted to reload the server. To do this, click Reload now.

After reloading the server, navigate back to OctoPrint Settings and the Plugin Manager. In the manager, search for "mqtt subscribe". The MQTT Subscribe plugin should now appear under Installed Plugins.