If you are trying to re-implement the ChirpSignal example you need to copy the Chirp function (in IntroductoryExamples.ExternalFunctions) too. If you look at the ChirpSignal model in text view you will see that the equation references the Chirp function:
block ChirpSignal "A block of an external chirp signal"
parameter Modelica.SIunits.AngularVelocity w_start = 0 "Angular velocity at start time";
parameter Modelica.SIunits.AngularVelocity w_end = 10 "Angular velocity at end time";
parameter Real A = 1 "amplitude of signal";
parameter Real M = 10 "time period for signal";
Modelica.Blocks.Interfaces.RealOutput u annotation(Placement(visible = true, transformation(origin = {140.0, -0.0}, extent = {{-10.0, -10.0}, {10.0, 10.0}}, rotation = 0), iconTransformation(origin = {107.5, 0.0}, extent = {{-7.5, -7.5}, {7.5, 7.5}}, rotation = 0)));
equation
u = IntroductoryExamples.ExternalFunctions.Chirp(w_start, w_end, A, M, time);
end ChirpSignal;
The simplest way would be to copy the entire package and then do your changes there. Please note that you need to update the referenced classes in the text view manually (e.g. changing IntroductoryExamples.ExternalFunctions.Chirp to YourNewPackageName.ExternalFunctions.Chirp).
Also, to get the model running properly you need to copy the Chirp.c file from the IntroductoryExamples folder to the folder where you save your new model or package, so that the WSM Kernel can find it.