As for where you get it, most hobby shops carry it. A 1lb. bag will run about 5 bucks, more than enough for 1-vehicle dio... As for how to use it on the Ferd, add a good-sized dollup of white glue to it, goop it around the suspension, and the lower part of the final drive cover... Use it inside fenders, on the hull above and behind the roadwheels, in between the track cleats, etc... Be careful with thickness though... 2 inches of mud in 1/35th scale is only about a 16th of an inch. Color it before you apply it, push some grass and pebbles into it, then after it dries, paint and drybrush it same as your groundwork... keep in mind that the tread of the roadwheels won't be caked in it, but will have some of the color of it, as will the inside of the track where it comes into contact with the roadwheels.
One of my old "mud" tricks was to use real mud, making a puddle (or waiting until it rained) in the ground a little bigger than the model and pushing the completed model down into the mud and swishing it a bit, to the depth that I wanted the vehicle to be muddied... Then I'd spray the dried mud with a heavy coat of clear varnish to lock it down. You can then paint it to look "wet"...