If the printer is appearing in the SELECT DYMO PRINTER drop-down in LIST settings and is not printing it is normally due to one of two errors:
1) you have more than one DYMO 450 printer driver (for this specific printer) installed on your computer which is confusing the system. To check this, go into your Printer Settings via Preferences (Mac) or the Start Menu (Windows), depending on the type of computer you are using, and see how many DYMO 450 printer drivers are installed. If there is more than one, delete them ALL and then re-install ONE driver via the correct links below.
Printer Driver Links:
Dymo 450 (and other models) Software Download Page
DYMO Software Mac, version 8.7
DMYO Software Windows, version 8.7
2) there is a problem with the printer itself. You can test this by trying to print a test label via the DYMO software that came with your printer. If it will not print a test label via the DYMO software then there is a problem with the printer and you need to contact DYMO support.
The latest version of the drivers may be found here: http://www.dymo.com/en-US