I'm havin problems with Micro Mark High Strenght Casting Resin CR-900
Aprox half four after I pour it in a mold it starts like boiling and expanding with a loooot of air bubbles that remain in it after it cures. I had some epoxy resins before but I bouht this one for casting thiner parts, but it doesnt work. On the label says that it is water-thin resin? I mix equal parts of both A and B
Does anyone knows what seems to be the problem?

Sounds like the resin is old. When it gets too old the chemical properties change and you get too much of a reaction, leaving you with the results you are describing. Try some new resin and that should solve the problem.

I agree with Gino.
Your resin is "expired"....you need a new one.
Resins have a "life" of about 6-8 months starting from the moment you open the bottle. From that moment they absorb oxigen and humidity from the air and this cause an alteration of the chemical properties at long distance. Judging by your words you use a gravity casting method without the help of a vacuum machine to extract the air bubbles trapped while you mix the two components...this is another reason of your poor results.
So if you haven't a vacuum machine I suggest you:
A) store the bottles in dry and fresh place (low humidity is very important), stir ALWAYS very well the components in the bottles with a stick before to use them and after this shake well the closed bottles.
B) pour the components in two separate small containers and wait for about 30 minutes....air bubbles will go up ( not all but.....)
C) mix slowly the components keeping the container inclined.
Of course for the best results a vacuum machine is a must.

Ciao
Gaetano