findpeaks only works on vector input; just use a loop and pass each column of your array in turn.
There's a fair amount of background information and examples of using findpeaks in the documentation; I'm unaware of any other tutorial but the web if full of stuff.
I'd suggest "just try it" and see what you get. If the spectra are only one-peaked, it seems it could hardly go wrong.
ADDENDUM
Since findpeaks can return anything from an empty set to some large number of peaks, you'll probably want to use a cell array to store the results initially or else wrap the call into a routine that processes the results of each column to ensure expected results rather than just presume every column will give one and only one peak. Of course, if the data are very clean, that may actually happen....I was rarely that lucky. :)
