You are looking for the property "svn: externals". See this svnbook section:
In the project that you want to use the shared project, then set a property in this directory with the name "svn: externals". This property contains the name of the directory that contains the external repository, and may have some other parameters so that you always get the same revision.
Example (from svnbook, which is a GREAT link for svn questions):
$ svn propget svn:externals calc
third-party/sounds http://svn.example.com/repos/sounds
third-party/skins -r148 http://svn.example.com/skinproj
third-party/skins/toolkit -r21 http://svn.example.com/skin-maker
, / http://svn.example.com/repos/sounds. -rNNN , , , , . , , , , .