How do I convert this ion exchange capacity into water softening capacity? I am researching for an extended experimental investigation into methods of water softening. One such method is the use of ion exchange resin. I have contacted the supplier of the resin for its exchange capacity, but I'm not sure how their answer relates to the removal of magnesium and calcium ions from water.

They say the exchange capacity is "1.0 mEq/mL min of 99% in H form."
In the H form there are one plus charged ions ($\ce{H+}$) being exchanged in the water softening process. But the species being exchanged for the $\ce{H+}$ ions are doubly charged ($\ce{Mg^2+}$ and $\ce{Ca^2+}$), so two $\ce{H+}$ ions must be exchanged per $\ce{Mg^2+}$ or $\ce{Ca^2+}$ ion.  

So, "1.0 mEq/mL min of 99% in H form" could equivalently be read as:  

The exchange resin has the ability to exchange $\pu{0.5mmol}$ of $\ce{Mg^2+}$ or $\ce{Ca^2+}$ ions per $\pu{mL}$ of resin.