Creating a 50hz comb filter in MATLAB
In order to use fdesign.comb you have to do something like the following to specify the parameters of the filter

D = fdesign.comb('notch','N,BW',8,.01); 


but then you must generate the filter as you mentioned in your question.

H = design(D); % H is a DFILT


you can then view the filter like this

fvtool(H)


One way to apply the filter to your data is to use the filter function

y=filter(H.Numerator,H.Denominator,inArray)


My favorite way to design a filter I've never used is with fdatool because you can play around with all of your parameters. Here is how I would do it:


Type fdatool at the command prompt and a window pops up
Specify Notching in the dropdown that initially says Differentiator
Inder IIR choose comb
Input your parameters
click Design Filter


Once you play around and it looks as you wish, you can either export the coefficients or generate the code that would create this filter.

To export coefficients


go to file and click export
make note of the variable names and click export
now you can use the filter command with y = filter(num,den,data);


To generate code


go to file and click generate MATLAB code
save the file


You see in this file something like the following

Fs = 48000;  % Sampling Frequency

N     = 10;    % Order
BW    = 1200;  % Bandwidth
Apass = 1;     % Bandwidth Attenuation

[b, a] = iircomb(N, BW/(Fs/2), Apass);
Hd     = dfilt.df2(b, a);


the line with iircomb is what generates the filter coefficients, you can now use y=filter(b,a,data);

Hope this helps.