Hi - I have an MC32 - I don't use it much...
The MC26 is a later model and I believe will be a little different to how I descibe below but similar.
Basically you can expand 2 of the receiver output channels into multichannel outputs (4 or 8) and once configured in ther handset software plug a multi unit (switcher 4 or 8 channel two state on/off switch or 4 channel porportional )into those sockets
On the MC32 you have 2 "Multichannels" (MUTLIC1 & MULTIC2) that you can configure(point to) (C5,C6,C7 or C8) output on the receiver. You can further set each on these Multichannels to be either a 4 or 8 function multichannel
A Nautic switching unit for the MC32 may have the number '16' in it's name but it's an 8 channel switch to confuse things
So using one of the 2 Mutlichannel software configurations - (MULTIC1) in the handset software, set to say Channel 5 you would plug the Nautic Multiswitch "Empflanger" socket into the receiver channel slot 5
This means the modified (Multichannel data stream) output of receiver channel 5 now goes into the Nautic Switch and we can switch multiple circuits.
The mapping of exactly what physical stick/switch/slider or pot to each of the Multichannels is performed in the handset software
Reading between the lines a new variant of the Nautic 16 switch does allow you to switch 16 circuits. I believe it has 2 inputs (A&B) - these would I think be 2 x 8 channels inputs
MULTIC1 8 channels - Mapped to Channel 5 - Plugged into Socket A
MULTIC2 8 channels - Mapped to Channel 6 - Plugged into Socket B
Would give you 16 switched channels.
Do you have 2 sockets on your switch unit marked A&B?
Update - not quite as explained - update to follow shortly!!
Here is a link for the multiswitch manual
Use patch cable between defined receiver channel (C5?) and X1 Slot 9 - it's possible the data for all 16 channels goes into this one slot rather than 2 slots on Nautic Switch!! or they would have provided you 2 patch cables!