If you want Realplayer to be the default application to open Smil file types:
Use the File Manager to navigate to a folder containing a Smil file. Click the file with the right mouse button, select Properties, and then the tab Open With. Click the radio button next to RealPlayer10, and close the dialog window.
Open Konqueror, and select Configure Konqueror in the Settings menu. Navigate to the File Associations section using the side bar, then type smil in the Find Filename Pattern box. Select the known type labeled smil and add Real Player 10 to the top of the Application Preference Order dialog.