Have you read the documentation for firlpnorm()? firlpnorm() seems like a sophisticated filter design for just a sine wave in additive noise, but here is an example.
Fs = 1000;
t = 0:1/Fs:1;
% 50-Hz sine wave in noise
x = cos(2*pi*50*t)+randn(size(t));
% lowpass filter
B = firlpnorm(22,[0 .15 .4 .5 1],[0 .4 .5 1],[1 1 0 0 0]);
% Filter magnitude response
fvtool(B,1)
% Filter the data
y = filter(B,1,x);
