Just avoid the construct f[var] in the definition of myPlot:
myPlot[f_, var_, a_, b_] :=
Manipulate[Plot[f, {var, a, b},
Epilog -> {PointSize[.02], Red,
Point[{pnt, f /. var -> pnt}]}],
{pnt, a, b}]
This way you can use myPlot with all kinds of functions:
myPlot[x^3, x, 1, 2]
myf[x_] := x^2;
myPlot[myf[x], x, 1, 2]
ff[x_, t_, z_] := x^3/t + z;
myPlot[ff[x, 3, 7], x, 1, 2]