Connect your printer via USB cable and turn it on. When opening MatterControl you will see a drop-down menu at the top of the window. Select "Add printer" or "Printers..." If you have connected to this printer before, you should be able to select it from the list in the pop-up window (which will disappear after selection) and it will appear in the grey bar of the application's screen. To complete the connection process, select "Connect" in the top left corner.
If you are unable to connect for any reason - ensure that your printer is turned on and connected, select "Printers...", select the pencil icon next to your recently added printer, and double check the settings.
This will bring you to a page where you can edit the name, make, and model of the selected printer - similar to the same page where you first set up the printer. Under the section that says "Serial Port", turn on the printer, take out the USB plug, and hit "Refresh"- you will see the USB port where it was become grey. Then plug it in, hit "Refresh", then select the port that became highlighted. Hit "Save", then it will take you back to the screen where you select a printer. Select the printer, which will take you back to the main screen, then click "Connect" in the top left corner.