After hooking up your printer, you need to install the drivers. If they did not come with the printer, you can find them here:
http://www.dymo.com/en-US/labelwriter-4xl-label-printer
After the drivers are installed, go to Devices and Printers.
Right-click on the Dymo printer, and select Printing Preferences.
Once that is opened, click on the Advanced button in the bottom right of the window.
Make sure that you have 1744907 4 in x 6 in selected.