If you are running Ubuntu 8.04 and you install Eclipse 3.4 with subclipse 1.4, you will probably get the "unable to load default svn client" error when trying to add a repository.
Here it is how I solved this problem. I assume that you have the 'subversion' and 'libsvn-java' packages already installed (if not, please do install them, and then follow the instructions).
1) Go to System --> Administration --> Software Sources
and under the "Updates" tab, check the hardy-backports updates. Close the window and update the package lists.
2) Run the update manager and update all packages (subversion will be updated from version 1.4 to version 1.5)
3) Edit the eclipse.ini file (it is located into the directory where you installed eclipse) by adding the following two lines, immediately after the "-vmargs" line
This should do the job, next time you start eclipse, it should correctly add your existing repository.