There is a known bug with Adobe's Flash Player 10 plugin for Mac browsers, where it loses any manual linebreaks from pasted-in text:
http://bugs.adobe.com/jira/browse/FP-780
There is nothing we can do to fix or workaround the problem in the way it affects our Seller Admin. As soon as Adobe fixes this bug, linebreaks in text will suddenly start working fine again for Mac users who upgrade their Flash plugin to use the bugfixed version.
Meanwhile, you may need to borrow a PC to paste-in any text from another file, or just type in your text manually.