The spec on the opto-isolated inputs says 3-30 VDC. Your 2.5/2.7 falls outside that range. The spec on the AN1/AN2 analog inputs operating in Digital mode (the default mode) says 2.5-5 VDC for an OFF condition. Your 2.5/2.7 may be reliable but that is very close to the low end of the OFF range. I would put the analog inputs, AN1/AN2, into Analog mode and set the alarm trigger levels for the input to something that reliably differentiates the ON from the OFF condition of the AC unit. There are two alarm trigger values, one controls the ON to OFF transition point and one controls the OFF to ON transition point. Would require a little experimentation in setting the Alarm trigger values to be sure they are not on the fringe of a range. You will need the free Simplehomenet Utility Suite to set the AN1/AN2 inputs to analog mode and the alarm trigger values (or some other HA software that supports configuring an EZIO8SA).
EDIT: Just realized I did not say anything about connections. Not sure what you are looking for. If you have successfully connected to opto-isolated Inputs 1-3, you are familiar with the Input connections. GND is pin 9, AN1 is pin 10, AN2 is pin 11. With that information in the Quick Start Guide and on the label above the connector (I think) can you expand on what more you need in this area.
2ND EDIT: Hate to add to the questions but the Simplehomenet web site lists the EZIO8SA opto-isolated Input voltage range as 2-30 VDC rather than the 3-30 VDC in the Quick Start Guide. With your target as 2.5/2.7 that would be the difference between being able to use an opto-isolated Input or not. Suggest calling Simplehomenet support to determine what the correct range actually is. You can still use the Analog input configuration with the trigger levels.