You are loading your mat file variables into a structure you named ecg, so you must specify which variable in ecg to plot. Your data is stored in a variable called array. Additionally, array is a 5969x18000 array. Plotting almost 6,000 waveforms on the same figure is not an effective way to view this data. You will also likely get some warning messages trying to plot 107.5 million points. Consider plotting just one for now.
Each ECG waveform is stored in a row. Try this.
ecg=load('train.mat');
ecg1 = ecg.array(1,:);
f_s=300;
N=length(ecg1)
t=(0:N-1)/f_s; % time period(total sample/Fs )
figure
plot(t,ecg1);
