Java moss is one of those plants that always stumped me, even though everyone says it is the easiest to grow. Here are my 2 cents. First make sure what you have is the "real deal" many places sell other mosses as Java mosss. If you have the real thing, you can try this,It depends on how porous the ceramic log is but, try using fine monofilament fishing line to attach clumps of the moss to the log. You migh also try adding Co2 injection to the aquarium set up to get a "jump start" (use a 3 litre soda bottle, water and yeast, check the many aquatic gardener sites for more info, the krib.com is one of the best IMO)
Again, just my $.02
Take care and good luck,
Dave