I am assuming that you are following the experiment given here http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p009.shtml#procedure
If you are and you have the correct measurements for the angles then this really boils down to a simple problem of substituting in the correct values of the different variables into Snell's Law.
What you have to do is to recognize that the laser is passing through two mediums i.e. air and jello (we will ignore the plastic container unless you made the jello thick enough to stand on its own in which case you don't need the container).
Now Snell's Law really boils down to the algebra because we just need to find one variable i.e. the speed of light in jello.
You did correctly state Snell's Law and the form you will need it in is
Velocity in jello = [sin(refracted angle in jello)] /[sin(incident angle in air)]* (velocity in air)
This comes from doing a little bit of derivation from the original Snell's Law and recognizing that the refractive index is the ratio of the velocity of light in the medium to the velocity of light in vacuum (which is a universal constant). I encourage you to do that bit of math and find out for yourself.
Now your measurements will give you the refracted and the incident angles and you can look up the velocity of light in air at different temperatures online or in a book. With that information you should have no trouble repeating this experiment multiple times with jello's of different density and finding the relationship between density of the medium and the speed of light in that medium.
Also, by your post I get the feeling that you are a little confused about the incident and the refracted angles so let me clear that for you if it isn't clear. The incident angle is the angle the laser makes with the normal to the interface in the air
while the refracted angle is the angle the laser makes with the same normal as before in the jello
Hope this helps and let me know if you have any more questions.