Hi @Sandy
Based on your image, the MRAC block should replace the Sum and PID controller blocks. If the link length is unknown, do you really need to estimate the link length in order to compute the MRAC output u? If so, you should design for Indirect MRAC, as mentioned in the documentation.
However, in a real scenario, it is very unlikely that measuring the link length of the robotic arm would be impossible, as all dimensions must be indicated in the technical blueprints for approval by the Project Manager and the Professional Engineer.
In my opinion, the best way to learn the adaptive control is through understanding the equations in the adaptive control books, or learn from the professor who teach the adaptive control course in that particular semester. No one ever truly learns adaptive control just by interconnecting the signals from ports to ports in Simulink and click the Run 
button.
button.
