Adding freon to a sealed system is a job for a pro.
Also there is no use adding freon until the leak is found and repaired.
This will be an expensive repair and often it makes more sense to put the dollars towards a new unit.

Check that the compressor is running. You can usually hear it but also should be able to feel it vibrating.

I am assuming from your post that you are not getting any cooling at all so the freezer is at room temperature or warmer. This points towards a sealed system problem.
If there is some cooling but the freezer is just too warm then it may be another problem (not the sealed system)

With no cooling but the compressor running it can be a number of things:
1. Low or out of freon.
2. A bad compressor.
3. A plugged sealed system.

Check your warranty as many units have a longer warranty on the sealed system.