The Mix6To2 plugin doesn’t have independent panning for each incoming channel. Instead, each incoming channel has separate level faders, feeding the left and right of the Mix6To2’s stereo output, respectively (or, in the case of a 5.1 surround channel, there’d be 6 faders per channel). So that amounts to a “balance” control, really.
If, in the stereo version, you just had the Left fader up (at 0db) on the “L” section, and just the Right fader up (at 0db) on the “R” section, then the incoming stereo would just pass through unchanged.
So, for obtaining mono from the incoming stereo, the Left input from the channel is routed, in equal proportions, to both faders of the section of the Mix6To2 marked “L”, and the Right input from the channel is routed to both faders of the section marked “R”. The two faders of the L section, and the two faders of the R section are all turned down to -6db, to satisfy the perceived pan law.