Ah yes - that piece of information might have been useful a few days ago ... ;)
Anyway, in your report design, select the field in question, and try something like this for the format :
$#,##0.00 ; [Red]-$#,##0.00
Or
0.00 ; [Red]-0.00
if you don't want the $ sign
Or
0 ; [Red] -0
if you...