Compile the svn-sources you have downloaded by right-clicking on "CodeBlocks.cbp" in the first "src" subdir and chosing "build".
Do the same with "ContribPlugins.workspace" if you also want to debug one of the plugins, or if you want to use one of them during debugging.
The run "update.bat" from the same directory.
Double-click "codeblocks.exe" from the "<C::B_source-root>\src\output" and open the C::B project file you used for compiling.
If it asks to change file-associations for c++-files you should not change actual associations.
That's it.
If you want to debug one of the contrib-plugins, open the "ContribPlugisn.workspace" and additionally open the "Codeblocks.cbp"-file.
To debug a plugin you have to activate the "Code::Blocks"-project and start debugger. If it asks which target to run, chose "src".
You can (of course) place breakpoints into the plugins-sources.