Hi Vincent,
In my understanding you want to use your custom distance function (dtwdist) with kmediod(). For this you don't need to use pdist function when calling kmedoid, You can simply pass the function handle of your custom function (dtwdist) and get your output.
The Name-Value pair 'Distance' only expect string or function handle. If using pdist, it gives an array as output which is invalid input for "Distance" name value pair. You can refer to following piece of code for refernce.
[IDX,C,sumd,D] = kmedoids(X,2,'Distance',@dtwdist);
Regards,
Anmol Dhiman