I had the same problem, which is strange as using MATLAB commands instead I could write arrays in the same kind of item, e.g.:
%% Initialization of an OPC client for data access by models
% (one array input)
Nsignals = 2; % number of signals array components
% Step 1 - Creating a data access client object, named 'da'
da = opcda('localhost','Matrikon.OPC.Simulation.1');
connect(da)
% Step 2 - Creating two groups in the client object, named 'grp'
grp = addgroup(da);
% Step 3 - creating one item in 'grp', with fully qualified item IDs
% given second argument of additem function
itm = additem(grp,'Bucket Brigade.ArrayOfReal8');
% Step 4 - reading the values of the two items of grp1
array_ini = zeros(1, Nsignals);
write(itm, array_ini);
r_grp = read(grp)
% Step 5 - disconnecting and saving the client for access from OPC Data Access Explorer and
disconnect(da)
save opcda_client da
