That's weird. It works for me. Given Rohit's comment, I think there must be some copy-paste weirdness. If you just use simple InputForm, I think you can avoid that:
u[x_, y_, m_] :=
Sum[(2/Sinh[n*Pi])*((1/(n*Pi)^2)*Sin[(n*Pi)/2] + (1/(n*Pi))*
Sin[(n*Pi)/2])*Sin[n*Pi*x]*Sinh[(1 - y)*n*Pi], {n, 1, m}];
Plot3D[u[x, y, 50], {x, 0, 1}, {y, 0, 1}]