As long as you're simply doing what you asked (to find a single scalar value in a vector), you can use any(A==B) like this:
A=[2 5 6 3 ];
B=3;
if any(A==B)
message = sprintf('The B value of %d is in A.', B);
uiwait(msgbox(message));
end
Needless to say, you can use the B1 variable instead of B.
