It should be okay to start it up.
After about 10 - 20 minutes, the plate should start to frost up from one side. Over time, the frosting will progress to the other side. If it does not frost up, ie only water
droplets appear on the plate, then you need to have it re-gassed.
Somewhere on the compressor
, it should state what sort of gas is being used. It will either be R12 or R134a. R12 is that nasty stuff that creates holes in the ozone. It is possible to re-gas it yourself but there are a few tricks eg you must ensure that no moisture gets into the system by purging the gauge manifold pipes. If you over-gas the system, you risk damaging your compressor. Re-gassing takes time because temperatures and pressures change as the system starts performing, so small adjustments to the gas pressure need to be made every 30 minutes or so.