Once your mold release is dry, go ahead and mix up some silicone and pour in a thin even stream into a corner of the mold until it's full to the top. I wrote a tutorial on silicone casting a while back that you might like to take a look at.
When your silicone is fully cured (this usually takes about 8 hours) demold it. You may have to get in there with an xacto to free up the little pyramids on the ends of those straight ribs, but those cuts will also help get the waxes out in the next step, so don't worry.