How to use low pass filter in Matlab for discrete data?
Check conv function. Use conv(f,x) where f is the filter and x is the signal.

You can also implement the filter process in frequency domain:

X=fft(x);
F=fft(f);
X_LP=real(ifft(X.*F));


You can use filter too. In y = filter(b,a,x), the filter is described by numerator coefficient vector b and denominator coefficient vector a,which is different in conv where the input is the time series of the filter. But conv(f,x) and filter(f,1,x) gives the same result in your case. 

EDIT

For dfilt.dffir type filter, use the code below (example page):

x = randn(100,1);    % Original signal.
b = fir1(50,.4);     % 50th-order linear-phase FIR filter.
hd = dfilt.dffir(b);    % Direct-form FIR implementation.
y = filter(hd,x);  


filter can handle FIR and IIR systems, while conv only works for FIR filter. 

Besides,  filter can also return the filter states, so that it can be used in subsequent calls without incurring filter transients.