I've asked Hikvision about this. Apparently, you need to use an older, customised version of HikCentral (V1.5.0081.20200330, as per the first guide attached) to be able to integrate the ADAM IO Modules to the VMS -
Hikvision Tech Support can provide the download file for this version. None of the other versions support the integration yet, but they are have let me know that this should not be the case for much longer.
Once you have this, the first PDF attached is a guide for connecting the modules to this version of HikCentral.
I've also attached the other guides that they gave me, which is a user manual for the ADAM-6000 series units, and a how-to guide for integrating them to an NVR, in case either are of any use.