Fitting Experimental Data with a Differential Equation

I would like to know how to define an ordinary differential equation(ODE) inside a curve fitting function. I'm a novice programmer in Igor, so I have been using the IntegrateODE command to check that I've written the correct ODE (e.g. Michaelis-Menten, see attached image ), but I still haven't been able to use it to fit any data set.

Function MM (pw,xx,yw,dydx)  // Michaelis-Menten relationship
//pw[0] = Volume Distribution (Vd)
//pw[1] = Maximal Elimination (Vmax)
//pw[2] = Drug Concentration (Km)

Wave pw, yw, dydx
Variable xx

dydx[0] = -pw[1]*yw[0]/(pw[0]*(pw[2]+yw[0]))

return 0
End

Any ideas on how to use this function as part of a Fitting function of the form

Function myFitFunc(pw, yw, xw) : FitFunc
WAVE pw, yw, xw
yw = ....
End

The experimental data that I'm trying to fit is for a titration calorimetry experiment that has three fitting parameters. The binding constant (K), the binding enthalpy (H), and the molar equivalents (n). In this case, the signal measured is the differential heat (dq/dXt) after each injection and it can be expressed as:

d_q/d_Xt = d_XM/d_Xt * H * n

for the equilibrium reaction X + M ---> XM. In this case, the ODE is derived from the mass balance equations and the binding constant as follows:

Xt = X + XM
Mt = M + XM
K = XM/(X*M)

Mt*Xt - (Mt-Xt+1/K)*XM + XM^2 = 0

which gives the ODE

d_XM/d_Xt = (Mt -XM)/ (2*XM-Mt-Xt-1/K)

Function  OneSite_ODE(pw, xx, yw, dydx)
Wave pw
Variable xx
Wave yw
Wave dydx

//pw[0] = Mt  total cocentration of  Macromolecule
//pw[1] = Kb binding constant

dydx[0]= (yw[0]-pw[0])/(2*yw[0]-pw[0]-xx-(1/pw[1]))

return 0
end

The OneSite_ODE has expected shape when Integrated an plotted against Xt. However, I was not able to fit my experimental data using the differential equation. This is the fitting function that I wrote, but it doesn't work.

Function OneSite(pw,wH,wX,wM )  : FitFunc  

wave pw, wX, wM, wH
//pw[0] = n
//pw[1] = K
//pw[2] = H
//pw[3] = Lo , hold constant
//pw[4] = Mo , hold constant
//pw[5] = Vo , hold constant

Duplicate/O wX  XM
Make/O/D/n=2 pODE

pODE[0]= pw[4]
pODE[1]= pw[1]

XM[0]=0
IntegrateODE OneSite_ODE, pODE , XM

wH = pw[0]*pw[2]*pw[5]*(XM-wM)/(2*XM-wX-wM-(1/pw[1]))  

End

Any help would be greatly appreciated!