Hi Chiara,
The error is caused because you need to convert the data to matrix format before you can add the two values as given in the following reference link- http://in.mathworks.com/help/finance/working-with-financial-time-series-objects.html
To add the two columns for each of the date values, you will have to use 'fts2mat' function- http://in.mathworks.com/help/finance/fts2mat.html
The following code snippet will help you sum the variables from 'Apple' with the 'Cat' values for each date-
SumVal= fts2mat(myfts.Apple) + fts2mat(myfts.Cat);
Here, I assume that the name of your financial series in 'myfts'. 'SumVal' will store the value of sum of the two columns.