How to convert the D'Arcy and Watt model from activity to concentration in mol/m^3
The general relation between activity (effective concentration) and concentration is as follows (see Wikipedia for more detail):
$$a_i = \gamma_i \frac{c_i}{c^\circ}$$
The standard concentration is defined as $c^\circ=1~\pu{mol dm^-3}$. For extremely dilute systems it is possible to just use the concentration instead of the activity (often used as a first order approximation, too):
$$\lim_{c_i\to0}\gamma_i\approx1 \implies a_i\to c_i$$
Depending on the accuracy necessary for your model, you might have to look up the specific values.

In any case you should again look up the definition of activity (or relative humidity) in D'Arcy and Watt's paper, because the activity defined for the above relation could be larger than 1.