Actually you should get line breaks using the enter key. If you mean line feeds like /n that probably will not work and are not necessary since that editor produces (x)html and clears out things itself. You probably can configure it differently. Please refer to the editor's documentation on http://wiki.moxiecode.com/index.php/TinyMCE:Index
There you also find a list of all supported browsers.
Also you can disable that editor by removing the file editor_config.js.php from zp-core/js.
Hum, sorry there was a typo in my previous message.
I meant "even if I choose html mode and add some < br />"
Acrylian, sorry but I have no line breaks with the enter key. This is automatically cleaned by the editor while saving.
Anyway :
Stephen, yes that did the trick ! I just added br => () in my allowed tags and now it works !
Thank you guys !