First off, thanks for asking for the clarification. I wasn't sure if my diagram or explanation would be that clear or not, and evidently they aren't. (For an excellent version of the diagram, check the ExamKrackers texbook).
The 'in' refers to inside a red blood cell, while the 'out' refers to outside a red blood cell, which can be plasma, extracellular space, cells, tissues, lungs, etc.
To clarify my reasoning, look specifically at this part of the diagram:
CO2 + H2O
-CA-> HCO3- + H+
In the tissues, CO2 is released into the bloodstream, causing this reaction to proceed to the right. (This is reversed in the lungs).
Recall that catalysts, and thus enzymes, only speed up the rate of a reaction, but don't affect the final equilibrium concentrations of products and reactants. In other words, an enzyme just brings a reaction to its equilibrium concentrations (much) faster than if there were no enzyme. However, even if the the enzyme is inhibited, or not even present, the system will still move towards equilibrium concentrations.
In the case of the bicarbonate buffering system in the tissues, CO2 is being released into the tissues. Under normal conditions, the CO2 is very rapidly converted to bicarbonate and protons by carbonic anhydrase, preventing a build-up in the concentration of CO2. If carbonic anhydrase is inhibited, however, the system still obeys Le Châtelier's Principle: CO2 is still reacting with H2O to become HCO3- and H+. The problem is that it doesn't react quickly enough, so the CO2 concentration builds up in the tissue (and consumes some H2O in the process; but this point is only minor. As you said @aldol16, water is the biological solvent, and it shouldn't really be considered to increase or decrease in concentration for the MCAT. This point is more so to help @BeerHelpsMeStudy understand why H2O wouldn't increase in concentration).
I hope my reasoning makes more sense now (and is correct).