Look at the filter function.
If you just need a 1-pole low-pass filter, it's
xfilt = filter(a, [1 a-1], x);
where a = T/τ, T = the time between samples, and τ (tau) is the filter time constant.
Here's the corresponding high-pass filter:
xfilt = filter([1-a a-1],[1 a-1], x);
If you need to design a filter, and have a license for the Signal Processing Toolbox, there's a bunch of functions, look at fvtool and fdatool.