To find the source address of a block to use with add_block, first browse the Simulink Library Browser to get to your block. For example, for the Counter block I browsed to DSP System Toolbox->Signal Management->Switches and Counters. Right-click the library in the browser and select to open it. In the new window, select the desired block, and then execute "gcb" in MATLAB to get its address. For the Coutner block, this produced "dspswit3/Counter", so then you can use a command like this to add it to a model named untitled:
add_block('dspswit3/Counter','untitled/mycounter');
