Chapter 4 of the Quartus II Version 7.2 Handbook: Volume 4 SOPC Builder (PDF) and online help indicates that Quartus® II user library paths can be used to specify the location of SOPC Builder components. This is incorrect. SOPC Builder components location can be specified using any of the following methods:
1) Specify the path using the Tools->Options->IP Search Path setting in the SOPC Builder.
2) Place your components directly in your Quartus II project directory.
3) Place your components in a sub-directory called "ip" that is located in your Quartus II project directory.