Just to clarify...
I'm thinking that the main misunderstanding is indicated by you (correctly) saying that "I would have thought that a "complete system" should work straight out of the box without the need for extra equipment
Indeed it should !
...And your excellent V55 would
, given that it's designed to be a "one for one", direct plug-in replacement for a microphone
In other words, it is meant for someone to remove their wired microphone ...and then plug the receiver in to that existing cable so that the microphone is now wireless.
The problem is that your other source (the PC for presentations) delivers its sound at a level that is much higher, "line" rather than "mic level" ...and that higher line level
is all that your existing passive mixer is designed to receive.
Which is why Sheriton has offered the excellent suggestion of replacing that existing passive mixer with one of these:http://www.thomann.d..._xenyx_802.htm
Once you do that, the instruction manual that arrives with it will give a clear explanation, so that you and your team will soon find it very easy to operate not only the XD-V55 systems, but also add other sources of sound such as iPods and the like.