No, that's not the problem. Though i'm pretty sure it's not necessary. The problem would be somewhere on that onclick. Check you have what ever scripts are necessary to call MM_openBrWindow. Someone posted something with that not too long ago, i forget if it was part of jQuery or not. Also check that preview_mailing.php is the correct url and that it's passing the right variables.
Disclaimer: I'm hungover 70% of the time i'm on here, any information given may not be correct, or even legible.