Thermal mastic is a heat transfer compound applied to the evaporator coils of the cooling unit at points where they make metal to metal contact wtih the freezer plate and fins of the refrigerator. Thermal mastic greatly enhances the cooling ability of the cooling unit, and failure to use it will result in poor cooling. As it turns out, thermal mastic is also a good sealer. If you have a tube of it, you can use the mastic to seal around the foam block of a cooling unit to prevent air leaks. The advantage of the mastic over other sealers like silicone is that it doesn't glue the cooling unit to the box, making it easier to pull the cooling unit at a later date.

Apply about a 3/8" bead of mastic along the entire length of the evaporator coil, as shown in the front view below.
When the freezer plate or fins are fastened snugly to the cooling unit, the mastic will spread to give a better cooling surface, as shown in the side view below. Do not use mastic to fill in the space of a poorly fitting connection -- correct the problem.
It is not unusual for the evaporator coil of the cooling unit to be recessed too far back into the foam block. It's not unusual, but it's also not good. In the example below, the foam block would need to be sanded or trimmed away along the dotted line and mastic applied, before the cooling unit would be able to do its job.