I presume you're using a Mac, as that sounds like the behavior of an old, known bug in Adobe's Flash Player plugin for Mac browsers that we posted about here:
http://www.e-junkie.com/bb/topic/3538
That bug was last reported in Flash plugin v10.3, and the current plugin is v11.5, so if they've eliminated the bug in recent versions, you might just need to update your Flash plugin to solve the problem:
http://get.adobe.com/flashplayer/
Reply here to let us know if updating Flash solved the problem for you, so we'd know for certain that Adobe has indeed fixed the bug in recent versions of Flash for Mac.
If that doesn't fix it for you, then the workaround we've recommended is to use the Mac's standard TextEdit program to compose your custom message (or any other newline-formatted text you want to enter in our Admin), then copy and paste your text from there into the relevant text field in Admin.