Many ways to do this:
c = {'hi'; 'my'; 'name'; 'is'}
lettertofind = 'i';
%method 1: plain character comparison
cellfun(@(s) any(s == lettertofind), c)
%method 2: strfind
cellfun(@(s) ~isempty(strfind(s, lettertofind)), c)
%method 3: ismember
cellfun(@(s) ismember(lettertofind, s), c)
%etc.
