It might help to check out my build log where I spent a fair bit of time with the SSR and how it was configured with lots of photos and decription. http://forums.reprap.org/read.php?178,702599,page=1
In short run small/low power cabling between the Duetwifi heated bed output and the DC inputs to the SSR make sure to get polarity the right way around.
On the AC side, use mains cables rated for at least the power rating of your heater preferably double that power rating . I.e. a 700w heater running at 240v pulls 2.91 amps, use 5A cable at least. If in doubt cannibalise a main cable of suitable rating. Take the neutral wire and run it to your heated bed mains heater, preferably adding a thermal fuse between the heater and the neutral wire, and ensuring you tuck it under the bed between the mains heater and whatever is above it (presumably aluminium plate), make sure its secure.
Now bring the live in from your power socket to one side of the SSR AC connectors. The other AC connector on the SSR goes to the live on your bed heater.
When a DC PWM or bang-bang "signal" is generated by the duetwifi, when the bed is on, the ssr will connect the AC. If its bang bang then it goes ON-OFF at full power in cycles lasting a second or two each when maintaining the bed temperature, if you send low frequency PWM (default setting for duetwifi) then it will pulse power at up to 10 hz to the bed for more precise control, the SSR will pulse connect the mains to the bed correspondingly..
The important part is the earthing. Make sure there is an earth wire from the aluminium bed, to the frame, and the to the SSR backing plate, and then out to your AC mains supply (preferably with an RCD/GFI) device. Ensure that all metal parts of the machine are grounded together, check this with a multimeter, aluminium frames are covered in aluminium oxide which is not very conductive so scratch the surface with the multimeter probe and check. The resistance between each of the metal parts should be low i.e. as close to 0 ohms as possible. Any poorly grounded or ungrouned metal parts need to be electrically joined together, you can use ring crimp connectors and mains wire and a drill and tap, with some m3 machine screws to earth bond the frame components together if needed. I didn't need to the tnuts and bolts created good connections.
If it stops controlling it for any reason the thermal fuse will trip permanently disconnecting the power, I chose 150 degrees, which allows a little headroom but should cut the power before anything melts or catches fire. If there is a wire break or the silicone heater (I assume you are using one) gets penetrated by anything, the RCD will trip.
As for your confusion, at any point that the duetwifi commands the bed to heat (that can be started by a gcode file, the paneldue, the web interface) then the heated bed output switches the SSR ON. When this signal goes off the SSR goes off. How it is wired doesn't affect how you tell it to come on/go off.
