Looking at the pages for all the required addons, there sure are a lot of them. However, if you just want to fly the probe, you don't actually need the Atlas 5 and Soyuz series to be able to fly it using the Delta IV Heavy Versatile (which doesn't require any other addons to work). Just use that and don't even install the other 2 launchers + their dependencies if you don't want them. I just did, here's how.
1. I made a clean install of orbiter by unzipping orbiter060929_base.zip into its own folder.
2. I installed Spacecraft2.dll and Multistage2.dll (I had a folder with all the different spacecraft.dll files, so I just copied all files in that folder into my orbiter folder)
3. Installed the Delta IV Heavy Versatile.
4. Installed the Chapman Probes addon.
5. Ran the scenario Chapman Orbiter/Chapman Launch (Delta IVH). It works.
After doing that, I installed the addon into my main Orbiter install, with which I use the
JoneSoft Generic Mod Enabler, by just putting the Delta IV Heavy Versatile and Chapman Probes addons into their own folders inside the MODS folder (I already had spacecraft3.dll, etc, installed), then enabling both of them in JSGME. It works in my main install too with lots of other addons.