YOu have to make sure you have the standard Nokia Pc Suite 6.85.14.1 installed, then open PC Suite (of course connect phone also to PC) - click on the envelope icon in PC Suit main view (NOkia PHone Browser opens and should open to your phone view and to messages subfolder i your phone.
Then on the picture panel in the middle of the Windows Explorer/Nokia Phone Browser view (assuming you have widows explorer settings so that you have selected "view folders" view, in the picture panel in the middle there is an envelope butoon, click on it to send an sms.
(check also PC Suite help, it explains also about how to send the sms's)