We use a Dymo 400 Turbo Label Printer and it seems to works okay with Simply. Did you choose the paper size correctly and make sure the Orientation is Landscape. then play around with the margins and fonts to get the desired placement on the label - start with the smallest font and zero margins
The only thing that you cannot do is use a custom designed label with your return address on it that you may have designed with the Dymo software. If you want to use a custom designed label, you could synch the Simply Vendor or Customer with Outlook and then print from Outlook - that works very well.
