From memory, if you go into the menu you will find a minimum shutter speed that you can use with the flash.... crank this down as low it will go. Exit the menu and then hold down the Flash control button to the side of the lens mount and select the slow setting. About this time if you set the camera to either P, A or S
mode you should get slow sync, providing you are in low enough light conditions.
The other option is simply to do it as the pros do it...
Set the camera to M for Madness (or manual, methodical, mystical... etc).
Set a slower shutter speed, relevant to the amount of light about and the subject you are photographing. (note, its a waste of time trying to attempt slow sync in daylight conditions)
Set and aperture that is going to underexpose the background a couple of stops (so that any brightness from the ambient light does not compound on the flash exposure)
Test the exposure without a flash if necessary
Turn on the flash and proceed to make photos, either in TTL
mode or Manual
mode.
REMEMBER... if you select a smaller aperture you also reduce the effective range of the flash. If you want to increase the range of the flash by double the distance you will need to open up the aperture two whole stops (i.e. from f11 to f5.6). In doing so (and to maintain the same ambient exposure entering the camera) you will need to speed up the shutter two whole stops.
hope this is a help!