Create a "buoyancy" force, which is based on the geometry of your blimp. Basically it is equal to the mass of fluid displaced by your blimp mutliplied by gravity. This means you need to find out the mass of air per unit volume at any particular altitude that your blimp will fly in, and multiply this density by the volume of your blimp. This will give you the mass of air displaced by your blimp, multiply this by gravity and apply the resulting force upwards at the centre of gravity of your blimp.
For a description of buoyancy have a look at this:http://scienceworld.wolfram.com/physics/BuoyancyForce.html