You should start by connecting or having connected a set of manifold gauges to see if you have a charge in the system. If you must, all you have to do is short the two connectors at the low pressure switch which will bypass the switch. Do this with the engine off and listen for the click. It could be that the engine must run for the CCU to trigger the connection. If this is the case, only touch the connectors together long enough to ensure that the clutch will work then disconnect them. DO NOT leave them together for any length of time because you may very well be running the compressor with no lubricant circulating.

A safer way to check the low pressure sensor is to simply use an Ohmmeter to see if it is closed. That is definitely the safest way.