To define a function you need an underscore after the variable. Also, the unknown function in the differential equation is u[x], not theta. This gives an answer:
Clear[x];
l = 0.5;
F = 13000/4;
f = 0.12;
Eel = 200000000000;
Iin = 0.05*(4*0.008)^3/4;
k[x_] = (2 f /l)/(1 + 4 (x/l)^2 (f/l)^2)^(3/2);
dsdx[x_] = Sqrt[(1 + 4 (x/l)^2 (f/l)^2)];
M[x_] = -F x - F l /2;
\[Theta][x_] = (u'[x]/dsdx[x] - (k'[x] u'[x])/(dsdx[x] k[x]))/k[x] +
k[x]/u[x];
NDSolve[{M[x] - Eel Iin \[Theta]'[x] == 0, u[0] == 1, u'[0] == 0},
u[x], {x, 0, 10}]
