how do i compute the mean-square in matlab?

I guess what you need might be

>> mean((l-k).^2)
ans =  0.10945

Data (You need ... for line continuation if you have data in different lines for l)

l=[0.02817088, -0.74100320, -0.54062120, -0.24612808,  0.06945337, -0.58415690, -0.51238549, ...
 -0.07862326, -0.42417337, -0.33482340, -0.21339753, -0.03890844, -0.59325371,  0.28154593, ...
   -0.32133359,-0.13534792,  0.14060645,  0.32204972,  0.44438052, -0.21750973,-0.59107599, ...
 -0.60809913]'

k= -0.2224834