Are you certain that the error message said something about "Open Source File"? I'm guessing it said something else.
Did the error message say "The installation of the ClickOnce solution failed with an exit code: -400"? If so, try this:
1. Make sure you have all copies of Excel closed.
2. In Control Panel, Programs and Features, search for and uninstall all programs that have NodeXL in their name. You can find all such programs using the search box in the upper-right corner of the Programs and Features window.
3. In Windows Explorer, see if this folder exists: “C:\Program Files (x86)\Microsoft Research\Microsoft NodeXL Excel Template.” If it exists, delete it.
4. Run the NodeXL setup program again.
If the error message isn't the "ClickOnce" message above, tell me exactly what the error message says, or at least the first sentence or two.