It bears repeating what samiam4 says. You must first establish a vacuum, connect your freon can, purge the feed line, attach the line to your vacuum gauge set, and commence filling by holding the can vertical with the orifice on top. The vacuum will suck in the first of the gas, which you can help by holding the can in a warm (not hot) water bath like this. This procedure is specifically addressed in the MB factory manual.

After the first of the gas is sucked in you can switch on the engine with a/c on full, temp on minimum and you will hear the compressor clutch click (even over the din of a diesel) as the compressor kicks in and more gas is sucked in.

Remember to charge only through the low side - the high side valve on your vacuum gauge set should be closed once your vacuum is established, and remained closed throughout charging. Disregarding this instruction can be life threatening.