<<xlr8r.m

model={{(Overscript[A⇄B, ℰ]),a,d,k}};
sim=run[model,
"TimeSpan"-> 5,
"IC"-> {A-> 1,B-> 0,ℰ-> 1}, 
"Parameters"-> {a-> 1, d-> .1, k-> 2}];
runPlot[sim]

Example with Boundary Conditions

f[t_]:=Which[t<5,0,t<6,1,t>6,0];
Plot[f[t],{t,0,10}]

model={
{A+ℰ-> AE,a}, 
{AE-> A+ℰ, d}, 
{AE->  B+ℰ, k}};
sim=run[model,
"TimeSpan"-> 10,
"IC"-> {A-> 1,B-> 0}, 
"Parameters"-> {a-> 1, d-> .1, k-> 2},
"BoundaryConditions"->{ℰ[t]->f[t]}];
Show[
runPlot[sim],
Plot[f[t],{t,0,10}, 
PlotStyle-> {Orange, Dotted}]]