To start Eclipse inside Eclipse you define a new "Eclipse Application" launcher and choose "Run a product" on the launcher "Main" page and select "org.eclipse.sdk.ide" in the combo box. Please clarify your question if this is not what you want.
The filename you gave looks like this is the ZIP from the "Eclipse Classic" download. To import projects from this into your workspace you would first have to unzip it somewhere then use "Import->Archive" on a plugin jar file to see the classes and be able to set breakpoints on them.
But: you say you want to debug the SVN plugin. If the file you have is in fact Eclipse Classic it will contain the Eclipse source alright but not the SVN team plugin (and the source for that). The easiest way to get this is to install "Subversive SVN Team Provider Sources" from the "Help->Install New Software..." dialog. Then you can use the "Import->Archive" on the plugin jar (you wont need the SDK-ZIP for this).
(edited in response to comments)