cd into the directory where you want to add the external project, then type:

svn propedit svn:externals .

Once the editor opens, add the name of the directory you want to add the external project into, and the URL of the project. So, if I want to add the UcsfLdapPHP repo (but without the PHP at the end), then type the following:

UcsfLdap https://svn.library.ucsf.edu/repos/UcsfLdapPHP/trunk

Once you save the file, svn will output something like:

Set new value for property 'svn:externals' on '.'

At this point, you need to commit the current directory:

svn commit -m "Added UcsfLdap as an external project." .

Finally, do the following to check out the code:

svn update

Now the external project is part of the current project.

See http://www.escapekeys.com/blog/2007/9/23/Subversion-Tip-How-to-Add-External-Repos-To-Your-Project