You need to modify the SOPC IP search path so that it can find your DSPB design.
Open SOPC Builder and click Tools->Options...
Select IP Search Path and add the directory containing the mdl file for DSP Builder standard (you may have to generate it first so that there is an MDLXML file as well) or the rtl destination directory for DSP Builder Advanced.
You should then see a new component appear in the Component library, which you can add to your SOPC design.
