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:
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:
Now the external project is part of the current project.