Resistance of mosfet's internal diode
For your purposes, there is no such thing as "a" diode resistance.

When you plot current vs voltage for a resistor, you get a straight line, and the resistance is simply the slope of the line. Well, actually that's conductance, and resistance is the reciprocal of conductance, but I hope you get the point. Resistance does not change with voltage or current.

Diodes are different. If you plot current vs voltage, you get an exponential curve, and it should be obvious that the slope of this curve depends on exactly where you pick to measure - and it varies enormously over quite a small voltage range.

So instead of resistance, you can talk about incremental or local resistance, which is specific to a single voltage or current, and is found by measuring the slope of the V-I curve at the specified level. It will not be accurate over a wide range, but can be useful when analyzing small changes. This is commonly done with zener diodes, for instance.

Or, you can model the V-I relationship as a simple resistor as long as you know the current you'll be operating at. In your case, if you want accurate numbers you're going to have to measure them yourself, or you can look at the data sheets for Vds vs Is. Divide the two and you'll have your number for that current. As a quick example, your figure shows Vos as -1.2 volts for Is of -2.5 amps, which means that, at 2.5 amps, it behaves like a 0.5 ohm resistor. Well, 0.48 if you want to get picky. 

The thing is, this will change with current and with temperature. Any curve you find in a data sheet will be "typical", and will often show those numbers for minimum, maximum and normal (room) temperatures.

And finally, just to make your life more difficult, for very large currents the bulk resistance of the diode will come to dominate the response, and the exponential curve will become a close approximation to linear. 

You can measure that, if you like, but you'll be getting close to whatever dissipation limits the FET has, and don't blame me if you Let The Magic Smoke Out.