Macbook not recognising Dymo 450 Turbo as a printer

I have found over the years that Demo's software is sometimes problematic. One thing I've found necessary is to follow their instructions and disconnect the power to the printer, install the software, then reconnect the printer. Finally, when they download their software, and you install it, they do not remove the old software app. You must go into the Applications folder and remove it to the trash yourself. If you still have the older app installed, possibly the computer is still opening it. Last, try restarting your computer and see if that helps. Sometimes it does. If not, then I would restart in the Safe mode, then see what happens. (Hold the Shift key down at startup.) In the past, I've had issues where the printer wouldn't print sometimes, requiring me to unplug the printer from power and then reconnect it. If you do this, be sure to wait until the LED on the printer goes out, which takes 15 or 20 seconds, before reconnecting it. Beyond that, you might check to see if Dymo has a user's help group on their web site. I hope this helps. Good luck.