Start with the correct characteristic equation:
1 + D(s)*G(s)*I(s) + D(s)*G(s)*Kt = 0 (not sure why the picture shows the last term divided by I(s) ).
Let L(s) be the "loop transfer function" that should be used to plot the root locus with respect to Kt. What is the CE written in terms of L(s) and Kt? Can the actual CE be manipulated into that form?